Preheat the oven to 170°c (150 fan/ Gas3/325F) and line a 20x30cm brownie tin with baking paper.
Whisk the eggs, sugars, and vanilla extract together. I like to use an electric whisk but you can do it by hand if you prefer - it will be a good arm workout! You're aiming for the mixture to become thick, pale, and fluffy. It usually takes about 3 minutes with my electric whisk.
Melt the butter and dark chocolate together in the microwave. Cook in 20-second bursts and stir each time, to prevent the chocolate from seizing. Allow the mixture to cool for a few minutes.
Mix together the egg mixture and melted chocolate. Be careful not to beat too much air out.
Fold in the flour and cocoa powder.
Mix in the frozen rolos.
Pour the mixture into a lined baking tin.
Dollop on the caramel and use a skewer or knife to make caramel swirls.
Bake in the oven for 35-40 minutes. Sprinkle with sea salt then place in the fridge for at least one hour to set firm.