Melt the butter in a small pan over medium heat and keep cooking, stirring now and then, until it foams, then the solids at the bottom turn hazelnut brown and it smells nutty (beurre noisette), about 4-5 minutes. Pour it into a cold bowl at once and let it cool until lukewarm.
Mix the ground almonds, icing sugar, flour and salt in a bowl.
Add the egg whites (no need to whisk them) and mix until smooth.
Finally stir in the lukewarm brown butter in a thin stream, including the browned bits at the bottom.
Preheat the oven to 180 °C, butter a financier or muffin tin and fill three-quarters full.
Bake for 12-15 minutes until the edges are golden and the centres spring back when pressed. Turn out after a few minutes and cool on a rack.
A Receptnaptár original recipe. Calories and protein are estimated.