For the cake

2 ½ oz. cocoa powder

5 oz. soft light brown sugar

13 fl. oz. water

7 oz. unsalted butter, softened

8 oz. caster sugar

3 eggs

12 oz. plain flour

1 tsp. baking powder

1 tsp. bicarbonate of soda

2 tsp. vanilla extract

For the frosting

12 oz. unsalted butter, grated and left to come up to room temperature

8 fl. oz. double cream

1 lb. dark chocolate (or 50% dark chocolate and 50% milk chocolate), roughly chopped

1. Preheat the oven to 180º Celsius. Grease and line a deep 9 x 13 inch baking pan with baking paper.
2. Sift the cocoa powder into a large bowl. Next add the soft sugar then carefully pour in the bowling water whilst whisking gently. Whisk well until thoroughly combined and the sugar has dissolved. Set aside.
3. In another large bowl, cream together the butter and caster sugar for 3-5 minutes or until pale and fluffy. 
4. Beat in the eggs one at a time ensuring they are each fully incorporated.
5. Sift in the plain flour along with the baking powder and bicarbonate of soda. Add the vanilla and gently pour half of the chocolate liquid into the same bowl. Mix until roughly combined. Next, add the remaining chocolate liquid and mix well until you get a smooth and slightly loose batter. Remember to scrape the sides and bottom of the bowl as you go along.
6. Pour the batter into the baking pan. Tap the tray a few times on a flat surface and bake in the centre of the oven for 25-30 minutes or until a toothpick inserted comes out clean. Once baked, remove from the oven and leave to cool on a wired rack.
7. Make the frosting. Place the chocolate in a large heatproof bowl and set aside. In a large sauce pan, heat the butter and cream over a medium heat until it the butter completely melts. Increase the heat until the liquid just starts to boil. When it reaches this point, immediately remove from the heat and gently pour the hot liquid into the bowl of chocolate. Use a whisk to combine the chocolate and dairy mixture into a smooth and glossy chocolate sauce. 
8. If the chocolate hasn’t thoroughly melted, place the bowl over a bain marie and continuously whisk until the chocolate has fully melted.
9. Leave the sauce to cool at room temperature. Place in the fridge for 7 minutes then stir well. Repeat this process about 4-5 times until you reach a thick yet silky consistency. If you have a very cold fridge leave in the fridge for 5-minute intervals.
10. To assemble the cake either spread or pipe the frosting on top of the cake. Cut into 12 equal slices and serve with icecream or whipped cream.