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 butter and sugar in a large bowl until fluffy, ideally using an electric mixer
Add the eggs, vanilla extract and buttermilk, and whisk until fully incorporated
Add the red food colouring and whisk until a strong bright red colour is achieved
Add the self raising flour, bicaebonate of soda and cocoa powder and whisk in until you can't see any flour anymore
Pour the batter into the lined traybake tin and bake for 25-30 minutes, or until a thin skewer inserted in the centre comes out clean. Leave to cool fully
To make the cream cheese frosting use an electric hand whisk to mix the butter on it's own briefly until loosened and creamy. Then add the icing sugar and mix it in. Finally add the cream cheese and vanilla extract and mix until smooth
Smooth the buttercream onto the sponge using a palette knife, then add the sprinkles to decorate (if using)
Cut into squares to serve. Store leftovers in an airtight container in the fridge and eat within 3 days
Notes
If you want to use plain flour (all purpose flour) instead of self raising, you will need to add 2 tsp baking powder to the recipe.