Place butter in a small sauce pan and place over medium heat. Melt and then cook until butter solids have turned brown but haven't burned. Keep a close eye on it because it happens quickly! Remove from heat and set aside to cool slightly.
Preheat oven to 375 degrees. Grease mini muffin tin with nonstick spray. Set aside.
Add almond flour, sugar, flour, and salt to a medium bowl. Whisk together. Add egg whites and almond extract. Whisk together to combine.
Add a little of the cooled browned butter to the batter to temper it, just in case it's still too warm. You don't want to shock the eggs with hot butter so tempering is important. Add the rest of the butter to the batter and stir together.
Using a small spoon, scoop batter into the prepared muffin tins, filling almost to the top. Decorate each filled tin with 4 sliced almonds, if desired. Place filled muffin tin into preheated oven to bake for 11-13 minutes. The edges will brown more than the centers so remove from oven before the edges turn black but are a deep golden brown.
Let cakes cool for 5-10 minutes before attempting to remove from muffin tin. Use a sharp knife and run along the edges of each tin to loosen up the cake from the tin. Remove and serve immediately. Cakes are best served on the same day but can easily be frozen and thawed for later consumption.