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 plant milk and leave to curdle.
Beat the vegan butter and sugar with an electric whisk until pale and creamy.
Add in the vanilla to the plant milk and whisk well. Don't worry if it is curdled and lumpy.
Fold in the flour, cocoa powder and bicarbonate of soda and mix until just combined and the batter is creating bubbles. If you find the batter is lumpy give it a quick whisk with the electric whisk again.
Divide between the cupcake cases, filling them two-thirds full.
Bake for 20 - 25 mins until 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.
Split the buttercream in half and add the yellow food gel to one half.
Fill a piping bag with a star nozzle so that one half is yellow and the other half is white.
Pipe swirls on top of the cooled cupcakes.
Top with half a creme egg.