First, we will make brown butter. This is optional (if you don't feel like it, you can just melt the butter) but I find that brown butter adds a nice nutty flour to your sponge. Cut your butter into small pieces and put it in a saucepan. At low-medium heat, melt the butter on the stove while stirring constantly until the butter changes color and starts to get a nutty smell. Don't get too far away from your butter, otherwise, it will burn! Pour into a bowl and add the honey onto it. Honey will help the butter go down in temperature.
In a separate bowl, whisk together flour, almond flour, and a pinch of salt.
Whip your egg whites to soft peaks with icing sugar. Add them to the flour mix gently, little by little, with a spatula to make sure the whites won't deflate.
Add the melted butter + honey to the mix.
Pour in a cake pan. Bake for 15 min at 200°C (392°F) or until golden brown.
Let it cool down, then cut off the border to get a 16 cm large sponge cake (you can use the 16 cm ring to do it). Wrap with saran wrap and leave it in the freezer.