Preheat the oven to 180C / 350F

Using an electric mixer beat the sugar and butter until well combined. Add the egg (keeping a tablespoon behind to brush the pastry), cognac, and vanilla extract, and mix well.

Mix the flours in by hand using a spatula. You should have a soft but fairly thick paste that holds its shape.

Place the sheet of puff pastry on a lined tray and spread the filling into the middle allowing about a 3cm border around. Top with ¾ of the sliced plums and then roughly fold the edges and corners around the filling leaving the center part open. Fill this with the remaining plum slices.

Brush the edges with the remaining egg and bake for 50 minutes until firm and golden brown. Loosely cover the pie with tin foil from about 30 minutes into the baking time to prevent over-browning.

Serve warm or at room temperature with a dusting of icing sugar, cream, or ice cream.