Step 1

Break up brownies into small pieces and place in the bottom of a 1.5 litre dish or 6 x 1 cup (250ml) serving glasses. Drizzle over the Kahlua, then set aside.

Step 2

Place dark chocolate in a heatproof bowl set over a saucepan of gently simmering water (don't let the bowl touch the water). Allow to melt, then stir very gently until smooth. Remove from heat and add 225ml boiling water, 1 tablespoon at a time, stirring to make a sauce (don't add water more quickly or the chocolate will 'seize' and become grainy). Pour sauce over the brownies, then cover and chill for 2 hours.

Step 3

Meanwhile, beat the egg yolks, sugar and cornflour together in a bowl with electric beaters until thick and pale.

Step 4

Heat 300ml of the cream in a saucepan over medium heat until just below boiling point. Pour the hot cream mixture over the egg mixture, stirring to combine. Transfer to a clean saucepan and place over low heat. Stir for 2-3 minutes until a thick custard forms.

Step 5

Place two thirds (100g) of the white chocolate in a heatproof bowl. Pour the custard into the bowl, stirring until the chocolate is melted and the mixture is well combined. Cool completely, then pour over the chocolate brownie base. Chill for 2 hours.

Step 6

Place the remaining 50g of white chocolate in a food processor with the walnuts and pulse until fine. Whip the remaining cream to soft peaks, stir in the walnut mixture, then spread over the trifle. Chill for at least 2 hours until set, then serve decorated with chocolate curls, if desired.