Czech sour cherry sponge cake (Bublanina)

1. Melt the butter and let it cool until lukewarm; line a baking tray of about 25 × 35 cm with baking paper. Stone the sour cherries and leave to drain (thaw and drain frozen ones first).

2. Separate the eggs. Beat the yolks with half the sugar and the scraped vanilla seeds until pale and fluffy, then add the lukewarm butter and the milk.

3. Whisk the whites with the salt and the remaining sugar to stiff peaks.

4. Gently fold the flour and the meringue into the yolk mixture alternately, a third at a time, so that the batter stays airy.

5. Spread the batter in the tray, scatter the cherries evenly over the top (they sink and the batter "bubbles" around them – hence the name) and bake in an oven preheated to 180 °C for 30-35 minutes until a skewer comes out clean.

6. Let cool, dust with icing sugar and serve cut into squares.

Tip: Bublanina (from "bublat", to bubble) is the quick bake of Czech summer gardens: it's always made with whatever fruit is in season – sour cherries, cherries, redcurrants, blueberries, apricots or plums all work.