Preheat the oven to 200C/180C Fan/Gas 6 and grease a non-stick 6-hole muffin tin with butter.
Melt the butter in a saucepan over a low heat. Transfer into a large, heatproof bowl to cool completely.
Once cool, mix in the flour, icing sugar, ground almonds, lemon zest and a pinch of salt.
Whisk the egg whites in a separate bowl using an electric whisk (or in a stand mixer) until frothy. Gently fold the egg whites into the flour mixture until you have a thin batter.
Pour the batter into the holes in the tin, filling each about three-quarters full.
Put 3 raspberries into each hole, then sprinkle over the pistachios.
Bake for 15–20 minutes, or until golden-brown and a skewer inserted into the centre comes out clean. You can finish with a sprinkling of icing sugar if you like. These are best served warm.
