Heat the butter in a skillet over medium heat until it melts. Reduce the heat to medium-low and swirl it in the pan as you wait for it to become a golden brown color. Keep an eye that the milk solids at the bottom of the pan do not turn black, as this will lead to a bitter flavor. Pour the browned butter into a bowl to cool down.
While the butter is cooling, sift together the flour, almond flour, and powdered sugar. Make a well in the middle of the dry ingredients and pour in the egg whites. Stir everything together until the dry ingredients are incorporated.
Pour in the cooled, browned butter and stir until it is all incorporated and the batter is silky and smooth.
Divide the batter evenly between the 12 wells of a greased muffin pan. Leave the batter plain or garnish each one as desired with sliced almonds, berries, or squares of chocolate.
Bake in a 400℉ / 200℃ pre-heated oven for 5 minutes. Then, reduce the heat to 350℉ / 180℃ and bake for 10 more minutes. Remove from the oven when they are golden brown and a toothpick inserted into them comes out clean. Let them cool in the pan for 15 minutes, then remove them and enjoy once at room temperature.
