Mix the dough. In a large bowl, combine the flour, water, molasses, active sourdough starter, and salt. Mix until no dry flour remains. The dough will be sticky and shaggy.
Bulk fermentation. Cover the bowl and let it ferment at room temperature for 8 to 12 hours, depending on your kitchen temperature. Warmer kitchens will ferment faster. The dough should roughly double in size and look puffy and airy. Note that the molasses may slow fermentation slightly, so don’t panic if it takes the full 12 hours.
Shape and proof. Once bulk fermentation is complete, gently shape the dough into a rough rectangle and place it in a parchment lined 9x13 inch pan. Cover and let it proof for another 2 to 3 hours until puffy and jiggly.
Prepare the coatings. While the dough is proofing, melt the butter and stir in the molasses and vanilla extract. Set aside. In a separate bowl, whisk together the dark brown sugar, cornstarch, ginger, cinnamon, cloves, nutmeg, and allspice. If using crystallized ginger, add it now.
Preheat oven. Preheat your oven to 425°F (220°C).
Tear the dough. Once the dough is fully proofed, use your hands to tear it into roughly 2 to 3 inch chunks. Don’t worry about making them perfect.
Dip and roll. Take each chunk and dip it into the molasses butter mixture, making sure it’s well coated. Then roll it in the spice sugar mixture until completely covered. Place each coated chunk back into the 9x13 pan as you go.
Dimple the dough. Use your fingertips to gently dimple the surface of the dough all over, creating small indentations. This pushes the spiced sugar down into the dough for beautiful marbled swirls and helps the focaccia maintain its texture while baking.
Bake. Bake for 25 to 30 minutes until golden brown and the sugar is caramelized in spots. The edges should be slightly crispy and the center soft.
Make the icing. While the focaccia bakes, whisk together the powdered sugar, cream (start with 30g and add more as needed), and vanilla until you reach a drizzly consistency.
Drizzle and serve. Let the focaccia cool for 5 to 10 minutes, then drizzle the icing over the top while it’s still warm. Serve immediately for maximum gooeyness.