Preheat oven to 180°C/160°C fan and line a 20cm x 20cm wide tin with parchment paper.
Prepare the brownie mix according to package instructions. Pour batter into the prepared tin and bake until a toothpick inserted into the middle comes out clean - this will be around 20 minutes. Let the brownie base cool completely in the tin.
Combine peanut butter and butter in a large bowl and beat with a hand mixer until smooth.
Add the icing sugar, vanilla, 2 tablespoons of milk and mix until smooth and fluffy. Add a third tablespoon of milk if the mixture looks a little too thick to be spreadable.
Spread the peanut butter mixture over the top of the cooled brownie.
Place both types of chocolate in a medium, microwave safe bowl along with the double cream. Microwave in 30 second bursts stirring in between until the cream is warm enough to melt the chocolate. Stir thoroughly until smooth.
Pour the chocolate topping over the peanut butter layer, making sure it covers the top evenly.
Place the tin in the fridge for 15-20 minutes until the topping has set. Carefully lift the brownie out of the tin and slice into 16 squares.
Store in an airtight container and eat within 3-4 days, or 7 days if kept in the fridge.
For more easy baking recipes using Lily O'Brien's Chocolate visit our Chocolate Blog