Quatre-quarts

Separate three eggs into two containers and weigh the whites and the yolks: 100 g and 60 g, so 160 g of egg in total. Any number of eggs works -- the weight is the measure.

Weigh out the sugar, butter and flour each to that same figure, 160 g.

Preheat the oven to 200C before you start anything else.

Rub butter over the whole inside of the mould by hand, then dust it with flour and tip the excess back out.

Melt the butter over very low heat -- melting only, no browning -- and turn it off.

Beat the egg whites with a tiny amount of salt and a squeeze of lemon on high speed until very firm.

Beat the yolks and sugar at a moderate pace until well incorporated and white.

Add the melted butter a little at a time, waiting for each addition to be fully incorporated before adding more.

Add the flour a little at a time. The batter stiffens; the liqueur loosens it back to the right consistency.

Fold the beaten whites in slowly. They are the raising agent here, in place of baking powder, and they lighten the batter. It is ready when it makes a ribbon.

Pour into the prepared mould, scraping everything in. It will not look full; it rises.

Bake 15 minutes at 200C, then drop to 180C for a further 25 minutes, 40 minutes in total. Use the oven without the fan.

A crack will form across the top near the end. That is normal.

Wait about five minutes, until the mould can be handled, then turn it out while still warm.

Wrapped in a towel it keeps up to three days and stays moist.