Heat the oven to 190C/170C Fan/Gas 5. Line a 20x30cm/8x12in baking tin with baking paper.
Gently melt the butter and the sugar together in a large pan. Once melted, take off the heat and add the chocolate. Stir until melted.
Beat in the eggs, then stir in the flour and the cocoa powder.
Pour the brownie batter into the prepared tin and bake for 30–35 minutes, or until the top of the brownie is just firm but there is still a gentle wobble in the middle.
Take out of the oven and leave to cool in the tin. Cut the brownies into 5cm/2in squares when only just warm, or completely cool.
For the caramel Sauce:
Tip the sugar into a heavy-based frying pan, stir in 4 tbsp water, then place over a medium heat until the sugar has dissolved. Turn up the heat and bubble for 4-5 mins until you have caramel. Take off the heat, then carefully stir in the cream and butter. Leave the sauce to cool, then tip into a squeezy bottle or jug for serving.