Slice the butter into 8 tablespoon pieces and place in a light colored saucepan. Melt the butter over medium heat (medium-low if your burners run hot).
Continue to cook while occasionally swirling/stirring the pan until the butter turns golden brown and smells nutty, about 8 minutes. Pour into a bowl and allow to cool for 15-20 minutes.
Preheat the oven to 375°F.
Grease the wells of two mini 12-muffin pans with soft butter. Be sure to get the whole well and the ridge along the top.
In a large bowl, whisk together the superfine sugar, almond flour, all-purpose flour and kosher salt.
Whisk the egg whites until they are foamy.
Add the egg whites, vanilla, and almond extracts to the dry ingredients, followed by the cooled brown butter. (It’s okay if the butter is still slightly warm). Whisk until the batter comes together. It will be a thicker batter.
Divide the batter between the prepared mini muffin pans. I use a 1 tablespoon scoop. If desired, top with sliced almond pieces.
Bake for 12-14 minutes or until the edges are golden brown and a toothpick inserted in the center comes out clean.
Let cool in the pan for 5 minutes. Then remove to a wire rack to cool completely.