1. Preheat oven to 180°C (160°C fan-forced). Grease and line a 20 x 20cm cake tin.
2. In a small saucepan, melt dark chocolate and butter over a low heat until smooth. Mix in sugar and set aside for 10 minutes. Add eggs and stir to combine. Transfer to a large bowl.
3. Sift flour into chocolate mixture. Fold in chopped milk and white chocolate. Pour into cake tin and smooth the top. Bake for 25-30 minutes or until firm. Cool completely in the tin.
4. Pretzel caramel topping: In a bowl, toss chocolate and pretzels. Arrange on half of the brownie and press down lightly. Bake for 8-10 minutes until chocolate melts. Leave to cool.
5. Popcorn topping: in a medium saucepan, melt butter and sugar on low, stirring until sugar dissolves. Increase heat to high and boil, without stirring, for 4-5 minutes until golden. Add popcorn and toss in caramel. Arrange on remaining side of brownie. Allow to cool.
Tip: If preferred, use milk chocoloate in place of dark.