Grease a round 28cm springform cake tin and add about 3 tablespoons of plain flour. Shake the tin so that the flour adheres to the butter on the sides and base. Discard any excess flour.
Preheat the oven to 190°C/Gas Mark 5.
Whisk the egg yolks with the sugar in a large bowl until the mixture is pale and creamy.
Add the citrus zest, almond flavouring, and a pinch of salt, then fold in the ground almonds using a large metal spoon.
In a clean mixing bowl, beat the egg whites until stiff. Fold one-third of the egg whites into the mixture, then carefully fold in the remainder.
Pour the mixture into the prepared tin and bake for 30–35 minutes, until the centre feels firm to the touch and the cake begins to pull away from the sides of the tin. Allow the cake to cool slightly before removing it from the tin and placing it on a wire rack to cool completely.
Traditionally, the cake is decorated with icing sugar in the shape of the Cross of Santiago (St James's Cross). Alternatively, you can simply dust it with icing sugar. Serve with fruit compote and whipped cream.