Line a 23cm/9" square baking tin with parchment paper
In a large, heat-proof mixing bowl, melt the butter and chocolate over a pan of just simmering water, stirring until melted and combined. Don't allow the bottom of the bowl to come into contact with the hot water
Remove from the heat and gently stir in the sugar until dissolved
Slowly add the whisked eggs to the butter & chocolate mixture, stirring all the while
In a medium sized mixing bowl, combine the ground almonds, cornflour, beetroot powder and cocoa
Add to the chocolate mixture, folding together until evenly combined and smooth
Pour the mixture into the to the lined baking tin using the back of a spoon to even it out and get it into the corners
Separately, in a couple of heat/microwave-proof ramekins, gently warm the peanut butter and jelly (either for a few seconds in the microwave or in a small bowl set over a bowl of hot water). Add a few drops of vegetable oil to the peanut butter or water to the jelly if the consistency is too thick to pour
Use a skewer to swirl the peanut butter and jelly through the chocolate mixture
Bake in the centre of the oven for 25-30 minutes or until the brownies have begun to form a crust around the edges. The middle should still be squidgy
Allow to cool completely, still in its tin, on a wire rack before lifting out and slicing into squares