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 and caster sugar in a large bowl until fluffy, ideally using an electric mixer for around 2-3 minutes
Add the eggs and vanilla extract, and mix until fully incorporated
Add the buttermilk, self raising flour, cocoa powder, bicarbonate of soda and red food colouring and mix in until you can't see any flour anymore and everything is fully combined
Pour the batter into the lined traybake tin, spread it out into an even layer, 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. Do not over mix as it can become sloppy and you won't be able to thicken it again
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
Video
Notes
Make sure all your ingredients (particularly the butter, buttermilk and eggs) are at room temperature before you start baking.
I used unsalted butter for the buttercream, for the cake I used a baking spread.
You will need to use a professional grade gel food colouring to get the same shade of red that you can see in my photos. Brands I recommend are Pro Gel, Sugarflair and Colour Mill.
I prefer to use Philadelphia full fat cream cheese for the buttercream as it gives me the best results.
Although I provide cup measurements, I highly recommend weighing your ingredients out using digital kitchen scales. It is the most accurate way to measure ingredients and will ensure the best results. Digital scales are very low cost and can be purchased for around £12 ($16.50).
For teaspoon (tsp) and tablespoon (tbsp) measurements, please use measuring spoons and not the type of spoons you eat with. Again this will ensure accuracy and provide the best results.