Method

1. Beat the butter and sugar together using electric beaters or in a stand mixer until light and fluffy. Crack in 1 egg and separate the other, then add just the yolk (freeze the egg white for another recipe). Pour in the vanilla and beat until smooth and creamy.

2. Add the flour, bicarbonate of soda, a pinch of salt and the milk chocolate chips, then mix with a spatula to form a stiff dough (if the mixture feels dry, add a splash of milk). Split the dough into two balls – one roughly two thirds of the mixture and the other, a third. Pat into 2cm-thick discs, wrap and chill for at least 1 hr.

3. Lightly dust a clean work surface with flour and roll the larger disc out into a 23cm round. Use it to line a 20cm springform cake tin, using your fingers to press the dough 3cm up the sides of the tin.

4. Stir the chocolate spread with a spoon to soften it a little, then stir in the white chocolate chips. Spread most of the mixture over the cookie dough base. Press the chocolate bars deeply into the chocolate spread then use a small spatula to spread the remaining mixture over the bars, levelling off the surface, there should be around 1cm of cookie dough rising above the filling. Chill for 20 mins to firm up the filling.

5. Roll the remaining dough out into a 19cm disc and lay on top of the filling, inside the cookie dough rim. Use your fingers to smooth over the edges and enclose the filling. Chill for at least 1 hr.

6. Heat the oven to 180C/160C fan/gas mark 4. Put the cake tin on a baking tray. Bake for 50 mins-1 hr, covering loosely with foil once the top is golden brown. Allow the pie to cool to room temperature in the tin before carefully removing the ring and base, then chill for at least 1 hr before slicing.