Put the dark chocolate and butter in a large microwave safe bowl. Microwave in 30 second bursts, stirring between each burst, until melted. Avoid overcooking as this can negatively affect the chocolate.
Add the caster sugar. Stir through with a manual whisk - I say stir as you don’t want to add air but a whisk is still the best tool as it distributes everything quickly.
Add the eggs and vanilla. Stir with the whisk again.
Put the flour through a sieve into the bowl. Stir to combine.
Add the chopped peanuts and fold through.
Assemble the brownie
Pour half of the brownie batter into your prepared tin and level off.
Top with the chopped Snickers.
Pour the remaining brownie batter on top and level off.
Warm the peanut butter in the microwave for 15-30 seconds to loosen to a pourable consistency. You may need to warm the caramel slightly too.
Drizzle the peanut butter and the caramel all over the batter.
Swirl a little with the handle end of a spoon.
Bake for approximately 30-35 minutes until a skewer pushed into the centre comes out with sticky crumbs (bear in mind you may hit Mars Bar, so test in 2-3 places).
Allow to cool in the tin, then chill overnight to make them easy to slice.
Slide onto a serving board and slice in 12 pieces (in a pattern of 3 by 4).