Line a 12-hole cupcake tin with paper cases. Preheat the oven to 180°C/gas mark 4. Use an electric hand whisk to cream together the butter, sugar and vanilla extract in a large mixing bowl until light and fluffy. Beat in the eggs one at a time, adding a little flour with each egg. Sift over the remaining flour and baking powder, then gently fold in with a wooden spoon until the mixture is smooth. Divide the cake mix between the cupcake cases, bake for 25 minutes and allow the cupcakes to cool.
Meanwhile, bring a pan of water to a gentle simmer over a low heat, and place a heatproof bowl over the top, making sure it doesn’t touch the water. Break up the white chocolate, add to the bowl and let it melt slowly, stirring occasionally. When almost all of the chocolate has melted, remove from the heat and stir until completely melted. Allow to cool slightly.
Use an electric hand whisk to beat together the butter and half the icing sugar. Beat on a low setting at first, so the icing sugar doesn’t puff up and out of your bowl. Beat on a higher setting until smooth, then repeat with the rest of the icing sugar. Continue to beat for 1-2 minutes, until pale and fluffy. Stir the melted and cooled white chocolate through the buttercream.
Divide the buttercream in half and spoon into 2 bowls. Add 1 tsp blue colouring to one bowl, and the remaining blue colouring and the pink colouring to the other. Beat until the colour is completely combined, then spoon half of each icing into separate piping bags.
Snip the ends off each, leaving a 1cm-2cm hole. Place both bags into a third piping bag, also with the end snipped off and fitted with a star-shaped piping nozzle. Make sure both inner bags are pushed all the way down towards the nozzle. Squeeze the icing bags from the top to move the buttercream down towards the nozzle so that both colours will come through to create a two-colour effect.
Starting from the outside, squeeze star shapes on to the top of the cakes. Continue in a spiral towards the centre of each one. You only want one layer, but all gaps should be filled. Refill the piping bags with the remaining icing when they run out.
Rest a 16cm dry foam sphere (available from florists) on a flower pot so the bottom half is inside the pot. Insert 12 skewers, leaving 2.5cm-5cm gaps between them. Push the skewers into the foam sphere until only 3cm of the skewer is showing.
Press the cakes all the way down on the skewers. Wait until the buttercream has set, then scrunch up squares of green tissue paper, squeezing each one in from the middle. Place the tissue between the cupcakes, making sure the paper does not touch the cakes, and secure with cocktail sticks, if necessary.