Heat the oven to 180C/356 F/gas 4.
Add cupcake cases to 1x 12 hole or 2x 6 hole cupcake tins.
Add the lemon juice to the soy milk and leave to curdle.
Beat the butter and sugar with an electric whisk until pale and creamy.
Add in the vanilla, milk and food colouring and whisk well. Don't worry if it is curdled and lumpy.
Fold in the flour and bicarbonate of soda and mix until just combined and the batter is creating bubbles.
Divide between the cupcake cases, filling them two-thirds full.
Bake for 20 - 25 mins until golden and risen. They should be bouncy to the touch and a skewer should come out clean.
Leave to on a wire rack to cool.
To make the buttercream, beat the butter, icing sugar and vanilla with an electric whisk until pale and creamy. It is done when it no longer feels grainy.
Add in the black food colouring and beat well until it is an even shade of black.
Pipe the icing on top of the cakes and decorate with sprinkles.