When the pastry is cool, use a big, sharp knife and ruler to cut the pastry into two 10 × 24cm neat rectangles, and a third rectangle about 12 × 26cm. Divide each of the 10 × 24cm rectangles into six so you end up with twelve 10 × 4cm pieces. Mix the icing sugar for the icing with about 2½ tbsp water to make a thick, runny icing. Remove a couple of spoonfuls and mix with enough colouring to get a deep pink colour, then transfer this to a small piping bag. Spread the white icing over the 12 × 26cm pastry rectangle, then pipe across thin stripes of the pink icing. Use a skewer or toothpick to drag lines through the icing, at right angles to the pink stripes, to feather the icing. Leave in the fridge until set hard. Cover the other pastry with clingfilm to keep it crisp and fresh.