Prepare Pan. Generously oil a 9x13 inch pan or large rimmed baking sheet with avocado oil.
Prepare Cinnamon Swirl. Mix brown sugar, cinnamon, and melted butter into a paste. Set aside.
Transfer Dough. The dough should be bubbly and increased in size. With wet hands, gently transfer the dough to the center of the oiled pan.
Cinnamon Swirl. Gently stretch the dough out in the pan to make room for the cinnamon mixture. Evenly spread the cinnamon sugar paste over the surface of the stretched dough.
Envelope Fold in Pan. Fold the top edge of the dough down about 2/3 of the way toward the bottom. Then fold the bottom edge up and over the top fold, just like sealing an envelope. This will create beautiful cinnamon swirls throughout the dough.
Flip. Flip the folded dough seam side down in the pan.
Second Proof. Cover and let proof for 3 to 4 hours at room temperature until puffy and jiggly.
Baking
Preheat oven to 425°F 220°C during the last 30 minutes of proofing.
Dimple. Drizzle dough with avocado oil. Use your fingertips to create deep dimples all over the surface.
Top. Arrange banana slices over the surface. Sprinkle with brown sugar, cinnamon, and flaky salt.
Bake. 25 to 30 minutes until golden brown and internal temperature reaches 200°F 93°C.
Cool. Let cool in pan for 10 minutes, then transfer to a wire rack.
Vanilla Drizzle
Whisk powdered sugar, milk, vanilla, and salt until smooth. Adjust consistency with more milk if needed.
Drizzle over cooled focaccia just before serving.