In a medium-sized bowl, beat the butter and caster sugar together until smooth. Then mix in the vanilla extract and the egg.
Add in the flour and baking powder. Mix well and when it's starting to stick, use your hands to press everything together into a ball of dough.
Divide the dough into two equal-sized pieces. Add black food colouring to one piece and orange food colouring to the other, then use your (gloved) hands to gently knead it in until there are no streaks left.
Between two sheets of baking paper, roll the black dough into a rectangle that's around ¼-inch thick. Repeat with the orange dough, aiming to make it roughly the same size as the black one.
Use the baking paper to lift up the black dough and place it directly on top of the orange one. Trim off any uneven edges to make a neat rectangle.
Carefully roll the dough into a log shape, starting with the longest side closest to you. Loosely cover the log and refrigerate for 30 minutes.
While the dough is chilling, preheat your oven to 180°C/350℉ (160°C/320℉ for fan or convection ovens) and line a large tray with baking paper.
Remove the dough from the fridge. Use floss or a serrated knife to cut any uneven edges off the log then divide it into 15-18 equal-sized slices. They will look quite small but they'll puff up as they bake.
Place the cookies onto your lined tray and bake for around 10 minutes. They should be firm to the touch around the edges and soft in the middle. When they come out of the oven, immediately sprinkle the extra sugar on top.
Leave them on the tray to cool completely then enjoy!