Put the biscuits in a food processor and blitz to crumbs, or tip into a bowl and gently bash with the end of a rolling pin. Mix in the melted butter. Tip into a deep 26cm fluted tart tin, and use a spoon to press the crumbs into the base and up the sides of the tart tin making sure it's compact and there are no gaps. Leave to chill in the fridge.

Heat the oven to 180C/160C fan/gas 4. Melt the chocolate and butter in a bowl in the microwave in 30-second bursts, stirring regularly until smooth, or in a heatproof bowl set over a pan of simmering water (ensuring the bowl doesn’t touch the water). In another bowl, beat the sugar and eggs with an electric whisk for 4-5 mins until creamy.

Pour the chocolate mixture and tahini into the sugar and egg mixture. Sieve the flour, cocoa powder and 1/2 tsp salt over the wet ingredients, then gently fold everything together, being careful not to knock out too much air. Pour the brownie mixture into the tart case and put on a baking tray (some butter might ooze from the tin). Bake in the middle of the oven for 20 mins until just set – it should still have a wobble in the centre. Cool at room temperature for about 2 hrs.

Meanwhile, make the topping. Put the custard and chocolate in a pan and cook over a medium heat, stirring, until the chocolate has melted. Remove from the heat and set aside to cool.

Spread the chocolate custard over the brownie layer. Chill for 30 mins until firm.

Whip the cream, 1 tbsp tahini and icing sugar together until it holds soft peaks. Pile on top of the pie and decorate with the sesame brittle. Will keep chilled for up to three days.