Preheat the oven to 190ºC, 170ºC fan, Gas 5. Line a 12 hole cupcake tin with 12 cupcake paper cases.
Beat the butter and sugar until very soft and fluffy, beat in the eggs, with the flour, baking powder and cocoa powder.
Stir in the chocolate chips. Spoon the cake mixture between the 12 cupcake cases.
Bake for 20-25 minutes or until the cakes spring back to the touch. Remove from the oven and cool on a wire rack.
Meanwhile, whisk the butter and icing sugar together until very soft and fluffy. Add the rose water, colouring and milk and whisk again for a few minutes until very creamy.
Fit a rose petal nozzle into a large piping bag. Fill the piping bag with the buttercream. Holding the nozzle directly over the centre of the cake, pipe the icing with quite a strong pressure over the surface, to create a rose shape.
Recipe by Sally Mansfield for Sainsbury's