In the bowl of a stand mixer, combine sourdough starter, buttermilk, eggs, sugar, and flour. Mix on low speed with the dough hook until just combined into a shaggy dough (about 2 minutes).

Cover the bowl with a damp towel or plastic wrap and let rest for 30 minutes (autolyse). This allows the flour to fully hydrate and makes the dough easier to work with.

After 30 minutes, sprinkle the salt and vanilla over the dough. Mix on medium speed for 3-4 minutes until the dough starts to come together.

Add the softened butter one chunk at a time (about 1 tablespoon pieces), mixing well between each addition. This takes patience! Let each piece fully incorporate before adding the next.

Continue mixing on medium speed for 8-10 minutes until the dough is smooth, elastic, and pulls away from the sides of the bowl. It should be slightly tacky but not sticky. The windowpane test works here: stretch a small piece of dough—it should form a thin, translucent membrane without tearing.

Place dough in a lightly greased bowl, cover tightly, and let rise at room temperature (68-72°F) for 8-12 hours, or until doubled in size. I usually do this overnight.

If you’re short on time or just want to simplify the process, you can skip the autolyse rest. Simply add all ingredients (starter, buttermilk, eggs, sugar, flour, salt, and vanilla) to the mixer bowl at once. Mix on low speed until all the flour is absorbed and you have a shaggy dough, about 3-4 minutes. Then proceed directly to adding the butter one chunk at a time and continue with step 4 above. The autolyse creates a slightly more extensible dough that’s easier to roll out, but skipping it won’t ruin your brioche. It just might take an extra minute or two of kneading to develop the same smooth, elastic texture.

Let butter soften to room temperature until very soft but not melted. It should easily indent when you press it but still hold its shape.

Place the softened butter in a medium bowl. Using a fork or hand mixer, whip the butter (and vanilla if using) for 30-60 seconds until it’s fluffy, lighter in color, and increased in volume.

In a separate small bowl, mix together brown sugar, cinnamon, cornstarch, salt, and cardamom (if using).

Set aside until ready to assemble. If making ahead, store covered at room temperature for up to 24 hours.

Using an offset spatula or butter knife, choose one half of the dough and spread the whipped cinnamon filling evenly down that entire half. You want the filling to go all the way to one long edge (this creates the dramatic visual swirl when you coil it). Leave about 1 inch of clean dough on each short end, but the filling should reach completely to one long edge. The other half of the dough strip (lengthwise) should remain completely bare.

Now for the taco fold: Take the bare half of the dough (the side without filling) and fold it over onto the filled half, matching up the long edges. You’ve now created a filled tube where the filling is visible along one entire long edge. This is your visual swirl side! If the dough edges seal together and hide your filling, simply peel them apart gently to expose that cinnamon filling. You want that gorgeous swirl visible along the entire length.

Pinch the short ends closed firmly to prevent filling from escaping during baking.

Grease a 10-inch springform pan lightly with butter or neutral oil.

Cover the pan loosely with plastic wrap or plastic bag.

Let proof at room temperature until puffy and jiggly, about 3-4 hours depending on your kitchen temperature. How do you know it’s ready? The dough should look visibly puffy and increased in size. When you gently press the dough with a fingertip, it should slowly spring back but leave a slight indent. When you gently shake the pan, the whole swirl should jiggle slightly (this is the “jiggly test”!). Don’t rush this step. Under-proofed brioche will be dense; properly proofed brioche will be cloud-like.

Preheat your oven to 375°F (175°C).

Place a parchment-lined baking sheet on the rack directly below where your brioche will bake to catch any butter or filling that may bubble out during baking and save your oven from a mess. Some caramelization on the bottom is normal and delicious!

Gently brush the top of the proofed brioche with egg wash (1 egg beaten with 1 tablespoon water). This creates that gorgeous golden-brown shine.

Bake for 45-55 minutes, until deep golden brown on top. Start checking around the 45-minute mark. If the top is browning too quickly, tent it loosely with aluminum foil after about 30-35 minutes.

The brioche is done when an instant-read thermometer inserted into the center of the swirl reads 190°F. This ensures the interior is fully baked and not doughy.

Remove from the oven and let cool in the pan for 10-15 minutes. The brioche needs this time to set, if you remove the springform ring too early, it might collapse slightly.

Carefully remove the springform ring. You can leave the swirl on the base or use a large spatula to transfer it to a serving plate.

In a medium bowl, whisk together powdered sugar and salt.

Add vanilla extract and 30g heavy cream or milk.

Whisk vigorously until completely smooth. The glaze should be thick but pourable—it should ribbon off the whisk and slowly settle back into itself. If it’s too thick, add more cream 1 teaspoon at a time.

While the brioche is still slightly warm (not hot!), drizzle the vanilla glaze generously over the top in a back-and-forth motion. Let it pool into the swirls and drip down the sides.

Let the glaze sit at room temperature for 15-20 minutes. As it cools, it will set into a beautiful matte, crackly finish.

Slice into wedges and serve while still warm. Watch people’s faces when they pull apart those layers and see all that cinnamon filling!