Add butter to a saucepan and place over medium-high heat. In about 5 minutes, the butter will become brown, with a beautiful nutty aroma - don't stir it. Using a kitchen brush, gently brush the muffin pan with the brown butter. Place the muffin pan in the fridge for 5 minutes. The butter will harden, which will help prevent the muffins from sticking to the pan once baked. Preheat the oven to 220 °C / 430 °F.
Using a hand whisk, gently beat the egg whites until they become frothy - make sure not to overbeat them. In a separate bowl, stir to combine all-purpose flour, sugar, ground almonds, lemon zest, and a pinch of salt. Add frothed egg whites to the flour mixture and stir to combine to get a smooth batter. Gradually, in two additions, incorporate brown butter and vanilla to the flour mixture.
bake and serve
Spoon the muffin batter into muffin cups, filling them to about two thirds (2/3). Arrange three sour cherries or cherries on top of each muffin batter. Transfer to the preheated oven. Bake for 25 - 30 minutes at 190 °C / 375 °F or until a wooden pick inserted in the center of the muffin comes out clean. Transfer the baked muffins to a wire rack to cool completely.