Preheat the oven to 200°C and grease a 25cm pie dish with butter.
Heat gooseberries and sugar in a pan for 8 minutes until softened and let cool.
Roll two halves of pastry on a floured surface until 8cm wider than the dish.
Line the dish with pastry and fill with the gooseberry mixture, leaving a 2.5cm border.
Cover with the second pastry sheet, seal the edges with a fork, and brush with beaten egg.
Bake the pie for 30 minutes until golden-brown.
Whisk egg yolks and sugar in a bowl until fluffy.
Simmer milk, cream, champagne, and vanilla seeds in a pan.
Whisk the hot liquid into the egg mixture and cook over low heat until thick.
Slice the pie into wedges and serve with a pour of champagne custard.