Step 1
Preheat oven to 180°C. Grease the base and side of a 20cm (base measurement) round cake pan and line with baking paper.
Step 2
Prepare cake mix using eggs, butter and 175ml water, following packet directions. Pour into the prepared pan. Bake for 1 hour or until a skewer inserted in the centre comes out clean. Transfer to a wire rack to cool completely.
Step 3
Meanwhile, line a baking tray with baking paper. Place sugar in a medium saucepan over medium heat. Cook, stirring occasionally with a wooden spoon, for 5 mins or until sugar melts and caramelises. Remove from heat. Stir in the peanuts. Pour over the lined tray. Set aside for 30 mins to cool.
Step 4
Break the peanut brittle into large pieces. Reserve one-quarter. Finely chop remaining peanut brittle pieces.
Step 5
Use an electric mixer to whisk the cream and ¼ cup (60ml) liqueur, if using, in a bowl until soft peaks form.
Step 6
Use a large serrated knife to cut the chocolate cake into 3 even layers. Place 1 cake layer in the base of a 20cm (base measurement), 12-cup (3L) glass serving dish. Drizzle with one-third of remaining liqueur, if using. Spoon over half the peanut butter and spread evenly over the cake layer. Top with one-third of the cream mixture and one-third of the caramel topping. Sprinkle with half the chopped peanut brittle and half the chopped Snickers. Continue layering with the remaining cake layers, liqueur, if using, peanut butter, cream mixture, caramel topping, chopped peanut brittle and chopped Snickers, finishing with cream mixture.
Step 7
Decorate the trifle with sliced Snickers and reserved peanut brittle. Drizzle with remaining caramel topping.