Preheat the oven to 350F. Grease and flour your 12 square brownie pan or you can use a regular muffin pan. (I use Bakers Joy Spray and then use a pastry brush to make sure it gets in all the corners and up the sides)
In a small saucepan, melt the butter over medium heat until it turns golden brown and starts to smell nutty. Pour into a glass measuring cup or bowl, making sure to scrape in all the brown bits, and allow to cool slightly
In a mixing bowl, whisk together the powdered sugar, almond flour, AP flour and salt
Add the egg whites, vanilla, and almond extract and stir until combined. Add the cooled brown butter and stir with a spatula until the batter is smooth and silky
Spoon the batter in to the prepared pan, putting just under 3Tbsp of batter in each cavity. (They should be just under 1/2 full)
Sprinkle some sliced almonds in to each cavity on top of the batter
Bake for 15-18 minutes, until golden
Let them cool in then pan for about 10 minutes, then gently remove and place on a cooling rack
Dust with powdered sugar before serving