I baked this on valentine’s day for my husband, it is a recipe from Annie Bells book. The first time I baked this I used 1/2 tsp but I didn’t feel I got the right dusky red color. Since then I have found that I use a tad more than 1/2 tsp of red color paste to get the rich dusky red.  My suggestion would be to start with 1/2  teaspoon and see how you go.

Ingredients

 For the cake

• 120g unsalted butter diced
• 300g golden caster sugar
• 2 large eggs
• 300g plain flour
• 230g buttermilk
• 1 tsp find sea salt
• 1 tsp vanilla extract
• 20g cocoa powder, sifted
• Approx 1/2 tsp red food colour paste
• 1 tbsp white wine vinegar
• 1 tsp bicarbonate soda

For the frosting

• 180g unsalted butter softened
• 150g icing sugar,sifted
• 450g full fat cream cheese
• 1 tsp vanilla extract

Method
Preheat the oven to 170 deg C fan forced. Butter two or three (if you have them) 20cm non stick cake tins. Line the bases with baking paper.

To make the cake, cream the butter and sugar together for 3-4 minutes until light and fluffy. Add the eggs one at a time, scraping down the sides of the bowl between each addition. Add the flour in three batches alternating it with buttermilk so you start and end with flour. Next mix in the salt, vanilla and cocoa. Start to add the food coloring, a knife tip at a time, until the mixture is a dramatic dusky red. Mix the vinegar and bicarbonate soda together in a small bowl, they will fizz, and add to the batter.
If using three cake tins, divide the mixture between them, weighing them to ensure you get evenly thick sponges. (If using two tins, add a third of a mixture to each one, bake these and then bake the third one). Bake for 20-25 minutes or until shrinking  from the sides and firm when pressed in the centre. Leave to cool in tin for 10 minutes, then turn the cakes out onto a wire rack, remove the paper and leave to cool top upwards.