Karpatka (Polish custard slice)

Method

1. Heat the oven to 180ºC fan/gas 6. Grease and line 2 large baking sheets. For the choux, put the butter in a pan with 450g water. Sift the flour and baking powder into a large bowl. Bring the butter and water to the boil, simmer until the butter has melted, then tip in the flour and beat with a wooden spoon until the mix pulls away from the pan sides and is lump-free. Cool for 5 minutes, then beat in the eggs, bit by bit, until you have a stiff, glossy mixture (this bit is much easier in a food processor).
2. Divide the pastry in two and spread on each sheet to make 2 x 20cm x 30cm rectangles, trying to achieve as rough or uneven a surface as possible (so it rises unevenly as it bakes). Bake for 20-25 minutes (don’t open the oven or the pastry will deflate), then turn the oven to 150°C fan/gas 3½ and cook for 20 minutes more until well risen, crisp and golden. Leave to cool.
3. Meanwhile, make the filling. Bring the milk, butter, half the sugar and the vanilla to the boil in a pan. In a large bowl, whisk the egg yolks with the rest of the sugar until pale. Add both flours to the egg yolks and mix well. When the liquid is boiling, pour a little into the egg mixture and whisk until well combined, then whisk back into the rest of the milk/butter mixture. Bring back to the boil, then cook for 5 minutes, whisking constantly, until thickened and lump-free. Remove from the heat, cover the surface with a sheet of baking paper, then set aside to cool.
4. Spread the cooled custard over a sheet of choux, then top with the other piece (use the most uneven piece for the top). Dust well with icing sugar, then cut into squares to serve.