Instructions:
- Whisk together flour, baking powder, and salt in a medium bowl. Set aside.
- Beat softened butter and granulated sugar in a mixer until light and fluffy (about 3–4 minutes). Scrape down the sides.
- Beat in eggs one at a time, followed by vanilla extract, mixing until just combined.
- Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix.
- Divide the dough into 6 equal portions (reserving a slightly larger portion for the 'White' base). Place each smaller portion into separate bowls. Add a small amount of gel colouring to each bowl and mix gently until the colour is uniform. The reserved portion remains white.
- On a large sheet of plastic wrap, flatten the white dough into a rough rectangle. Lay the coloured dough sections side-by-side on top of the white base, pressing them together gently.
- Roll the coloured and white dough together using the plastic wrap to guide the roll, forming a tight, uniform log about 2-3 inches thick.
- Wrap the log tightly in the plastic wrap and chill in the refrigerator for a minimum of 2 hours. This is crucial for clean slices.
- Preheat oven to 350°F (175°C). Line baking sheets with parchment paper.
- Remove the chilled log. Slice thinly (about 1/4 inch or 6mm thick). Arrange slices onto prepared sheets, leaving 2 inches between each.
- Bake for 10–12 minutes, or until the edges are barely set and very lightly golden.
- Cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.