Brown the Butter: Melt 113g of unsalted butter in a heavy-bottomed saucepan over low heat until golden and fragrant, about 5-7 minutes.

Preheat Oven: Preheat your oven to 350°F (180°C) and prepare an 8x8-inch baking pan lined with parchment paper.

Combine Dry Ingredients: In a bowl, whisk together 300g gluten-free flour, 1 tsp baking soda, ½ tsp kosher salt, and ½ tsp cinnamon.

Mix Wet Ingredients: Cream the cooled butter with sugars and vanilla, then add eggs one at a time, and fold in mashed bananas.

Combine Mixtures: Gradually add the dry mixture to the wet mixture until just incorporated.

Bake: Pour the batter into the pan, smooth the top, and bake for 40-45 minutes until a toothpick comes out clean.

Cool Completely: Allow the cake to cool for 10 minutes in the pan, then transfer to a wire rack to cool completely.

Make Caramel Sauce: Combine sugar with warm water, stir until dissolved, heat until amber, then mix with heavy cream and butter.

Prepare Buttercream: Whisk egg whites and sugar over simmering water until 160°F, then add butter and caramel until creamy.

Frost: Spread the buttercream on the cooled cake and drizzle with additional caramel and sprinkle with flaky sea salt.