Preheat oven to 160°C / 140°C Fan / 325°F / Gas Mark 4. Line a 20cm heart-shaped tin with parchment paper on the base, and up the sides. Set the tin aside for later.
Melt the butter and chocolate over a bain-marie or in the microwave in 20-second intervals until completely melted and smooth. Set this mixture aside and leave it to cool for 5 minutes.
Meanwhile beat the eggs and egg yolks together until combined. Once the chocolate mixture has cooled slightly, add the egg mixture and stir to combine.
In large mixing bowl, mix together the flour and sugar, sift the cocoa powder over the top and mix until thoroughly combined and no lumps of flour remain. Now add the chocolate mixture and stir until combined, try not to over mix otherwise this may result in your baked brownie being tough. Transfer the brownie batter to the prepared tin and smooth the surface. Bake the brownies for 40-45 minutes, they’ll be ready when they’re risen, the top has started to slightly crack and a cake tester when inserted into the centre comes out with just a few moist crumbs attached - the brownie should still have a slight wobble in the centre.
Leave the brownie to cool completely in the tin, then once cooled leave them to set for at least 6 hours or overnight for best results.
Decorate further if desired by making a chocolate ganache topping. Chop the chocolate up into small pieces, and add to a small mixing bowl. Place the cream in a small saucepan on medium heat. Once the cream just starts to bubble, take off the heat and pour over the chocolate. Leave for a minute, then stir until you have a smooth, glossy mixture. Directly cover the top of the ganache with a piece of cling film, leave to set at room temperature for 1-2 hours.
To decorate, I filled a piping bag fitted with a nozzle with the cooled, thickened ganache. Piping around the edge with the ganache. I then decorated further with chocolate/sprinkles. The brownie will keep stored at room temperature for up to one week, if you can resist it!