Citrus and Seed Sandwich Cake

For the cake: heat the oven to 170°C (150° fan) 325°F gas 3. Line 2 x 23cm|9" deep loose-based cake tins with non-stick baking paper.

Sift the flour, 280g|1 1/4 cups sugar, baking powder and salt together twice into a mixing bowl.

Whisk together the egg yolks, water, oil, zest and vanilla until smooth. Stir into the flour mixture until smooth.

Whisk together the egg whites and cream of tartar until soft peaks form. Add the remaining sugar and whisk until stiff, but not dry.

Fold one-quarter of the egg whites into the egg yolk mixture, then fold in the remaining whites with the poppy seeds.

Divide between the tins and spread evenly.

Bake for 40-50 minutes, until the top springs back when lightly pressed and a skewer inserted into the centre comes out clean. Cool in the tins on a wire rack for at least 1 hour before removing from the tins.

For the filling: beat the butter until soft. Sift in the icing sugar and beat until until light and fluffy, then beat in the lemon juice and food colouring.

Turn one cake upside down and spread with the lemon buttercream. Place the other cake on top. Place on a serving plate.

For the buttercream: beat the butter and lemon zest until very soft and lighter in colour.

Sift in the icing sugar and add the lemon juice. Beat until incorporated and creamy. If necessary add a little milk and beat until blended. Beat in the food colouring.