In a small saucepan, melt 100g of unsalted butter over medium heat until it becomes foamy and turns golden brown, about 5-8 minutes. Let it cool slightly.

In a mixing bowl, whisk together 5 egg whites until frothy. Slowly incorporate 150g of icing sugar until dissolved. Mix in the cooled brown butter.

Divide the batter into three bowls and fold in 25g of almond flour, hazelnut flour, and pistachio flour into each, respectively, adding a pinch of salt to each.

Transfer each batter into piping bags and chill for about 1 hour.

Preheat your oven to 175°C (347°F). Brush your financier tin with melted butter.

Pipe the chilled batter into the molds, filling each cavity about three-quarters full. Optionally sprinkle with chopped nuts and bake for 20 minutes until golden brown.

Let the financiers cool in the tin for 5 minutes, then remove and cool completely on a wire rack.