Chocolate Mississippi mud pie

Method

1. For the base, grease a 23cm loose-bottomed cake tin with oil and line the base with baking paper. Whizz the Oreos and digestives in a food processor to fine crumbs. With the motor running, add the melted butter. Tip the mixture into the cake tin and pack it down, working it up the sides to just over halfway. Chill for at least 30 minutes until firm (or pop in the freezer for 15 minutes). Preheat the oven to 160°C/fan140°C/gas 3.
2. Put the cake tin on a baking sheet and bake in the preheated oven for 10 minutes. Remove and set aside to cool. Increase the oven temperature to 180°C/fan160°C/gas 4.
3. For the cake, melt the chocolate with the espresso powder, rum, butter and salt in a pan over a very low heat. When melted, whisk in the sugar.
4. Off the heat, beat in the eggs one at a time, then beat in the vanilla paste, golden syrup and finally the flour (don’t worry if it curdles – whisking will bring it together again).
5. Pour the batter into the cake tin, then bake for 30-35 minutes. Remove from the oven and cool in the tin.
6. For the mousse, put 250ml of the cream and the chocolate in a bowl over a pan of barely simmering water and heat gently until the chocolate is melted (don’t let the water touch the bowl). Stir, then remove from the heat and whisk in the egg yolks.
7. In a separate very clean bowl, whisk the egg whites with 30g of the icing sugar until just stiff, then fold into the chocolate mixture. Spoon over the cake and chill for 1-2 hours (or overnight, see Make Ahead) until set.
8. To serve, remove the pie from its tin. If it resists, you can run a cook’s blowtorch around the outside gently to loosen the mousse. Whip the remaining 300ml cream and 50g icing sugar to medium-stiff peaks, then dollop it onto the pie and sprinkle with grated chocolate. Leftover cake will keep in the fridge for 4 days.