Method
Brown the butter
Melt the butter in a light-coloured saucepan over medium heat. Keep going past melted — you want it to foam, then subside, then begin to smell nutty and show amber-coloured solids at the bottom. The moment it smells like toasted hazelnuts, pour it immediately into a bowl to stop the cooking. Let it cool to room temperature.
Mix the dry ingredients
Whisk together the icing sugar, flour, ground almonds, and salt in a large bowl. Make sure there are no lumps in the icing sugar — it tends to clump and you don't want pockets of sweetness.
Combine
Add the egg whites and vanilla to the dry ingredients and stir gently until just combined — don't whisk, don't beat, don't try to create volume. Then fold in the cooled brown butter (including the brown solids — that's the flavour) until you have a smooth, glossy batter. Refrigerate for at least 1 hour, ideally overnight.
Bake
Heat oven to 200°C (180°C fan). Butter your financier moulds generously. Fill each mould about three-quarters full — they puff. Bake for 11–13 minutes until the edges are deep golden and the centres spring back when pressed. The contrast between the crisp edge and soft centre is the whole point.
Unmould and finish
Leave in the tins for 5 minutes, then turn out onto a rack. While still warm, add a tiny pinch of flaky salt to each one. Eat within a few hours — they're best the day they're made, still slightly warm, with nothing alongside.
On resting the batter: The overnight rest isn't optional if you want proper domed tops. The hydration time changes the structure of the batter in a way that makes a real difference. Plan ahead.