Preheat. Set the oven to 390°F (200°C).
Brown the butter. Melt the butter in a saucepan over medium heat, stirring occasionally, until it turns golden brown and smells nutty — about 5–10 minutes. Transfer to a bowl and cool quickly by placing the bowl in cold water.
8 tbsp butter
Mix egg whites and sugar. Whisk the egg whites and sugar together just until a little foam forms on the surface. You're not whipping them — just combining.
5 egg whites, 7 tbsp sugar
Add honey and vanilla. Stir in the honey and vanilla extract until incorporated.
1⅓ tbsp honey, ½ tsp vanilla extract
Add dry ingredients. Sift the almond flour, cake flour, and baking powder together, then mix into the wet ingredients until smooth.
¾ cup almond flour, 9 tbsp cake flour, ½ tsp baking powder
Add the brown butter. Pour in the cooled brown butter including the browned bits at the bottom — they carry a lot of flavor. Make sure the butter isn't too hot before adding or it may cook the egg whites.
Prepare the pan. Line the bottom of the 8-inch springform pan with parchment. Grease the sides with butter and dust with powdered sugar for easy release.
powdered sugar
Fill and bake. Pour the batter in — it will be fairly fluid. Place in the oven and immediately reduce the heat to 360°F (180°C). Bake for 13–15 minutes, until the edges are lightly golden and the center is set.
Cool. Let the cake cool in the pan for at least 10–15 minutes before removing, then cool fully on a rack — about 1 hour. Once completely cool, use a long serrated knife to cut it in half horizontally into two even layers.