Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4, and grease and line a 12" x 9" traybake tin
Make the sponge by mixing the baking spread (or butter) and light brown soft sugar in a large bowl until fluffy, about 2-3 minutes, ideally using an electric mixer
Add the eggs, milk and vanilla extract, and whisk until fully incorporated
Gently whisk or fold in the self raising flour, cocoa powder, baking powder and bicarbonate of soda
Gently fold in the chocolate chips
Pour the mixture into the tin, smooth it out if needed to spread it out evenly
Bake for 25-30 minutes or until a skewer inserted in the centre comes out clean. Leave to cool fully either in the tin or on a cooling rack
To make the buttercream, start by melting the dark chocolate. I melt mine in the microwave, start with a 30 second blast, then stir, then continue with 10 second blasts, stirring between each one until it is fully melted. Set it aside to cool
Mix the butter in an electric mixer with the paddle attachment for a few minutes. Add the icing sugar, cocoa powder and double cream and mix until well combined
Add the cooled dark chocolate and mix in until combined
Spread or pipe the buttercream all over the cake
To serve, decorate as you prefer, then slice into squares. Store any leftovers in an airtight container in a cool place and eat within 3 days