Combine the flour, baking powder and a pinch of salt in a large bowl. Rub in the cold butter using your fingertips until the mixture resembles breadcrumbs. Stir in the sugar, egg and vanilla, and knead together until a dough forms. Divide into two equal portions, shape into discs, wrap and chill for at least 1 hr. Will keep chilled for up to 24 hours.

To make the pastry cream, heat the milk and vanilla in a saucepan until just simmering. Whisk the egg yolks, sugar and cornflour together in a heatproof bowl until pale and smooth, then slowly pour in the hot milk mixture, whisking continuously. Pour the mixture back into the pan and cook over a medium heat for 4-5 mins, stirring continuously until thickened. Remove from the heat and stir in the butter until smooth. Cover with a disc of baking parchment, ensuring it’s touching the surface of the cream, to prevent a skin from forming. Leave to cool.

Heat the oven to 180C/160C fan/gas 4 and butter a 22cm round tart tin. Roll out one of the pastry discs on a lightly floured surface until it’s large enough to line the base and side of the tin, then gently press it into the tin, leaving any excess overhanging the rim. Spread the jam over the base, if using, then beat the cooled pastry cream until smooth and carefully spread this over the jam layer (or the plain base). Roll out the second pastry disc as before, then gently lay it on top of the pastry cream, pressing the edges together to seal. Trim off the excess pastry and use a fork to crimp the edges. Brush over the beaten egg and press a fork into the surface to create a crosshatch pattern, if you like.

Bake for 40-45 mins, or until the pastry is golden brown. Cool completely in the tin before removing to a serving plate. Slice and serve at room temperature. Will keep chilled for up to two days.