Preheat your oven to 180C / 350F / Gas mark 4 and lightly grease and line a 9 inch square pan with baking parchment and place to one side.
In a large mixing bowl add in the flour, cocoa and both sugars. Give them a gentle whisk to combine them together.
Pour in the vanilla extract, vegetable oil and all the eggs. Mix everything together until you have a thick glossy batter.
Add in the salted peanuts and mini marshmallows and mix till evenly distributed.
Pour the batter into your prepare tin and pop it in the middle of your oven for 20 to 25 minutes.
Like most brownies the edges will be firmer than the middle when they are ready to come out of the oven.
Leave them in the tin too fully cool down.
If you keep your salted caramel in the fridge, then take it out an hour before needing it so it is easier to spread. As I mentioned above it doesn't matter if you use homemade or shop bought salted caramel sauce.
Once the brownie is cooled to room temperature lift it out of the pan and peel off any parchment paper still attached and place the brownie on a flat serving plate.
Gently spread your caramel sauce all over the top of your brownie taking it right to the edges. Place your peanuts in a small zip lock bag and give your peanuts a little bash breaking them up. Then sprinkle all over the top of your brownie. Cut up into 15 squares and serve.
Kept in an airtight tin these brownies will keep for up to 5 days.