Line a small tray with parchment or a silicone mat. Line the countertop with a silicone baking mat or silicone rolling mat.

In a medium bowl, whisk together all purpose flour, baking powder, and fine sea salt. Set aside.

In a large mixing bowl or the bowl of a stand mixer, mix butter and granulated sugar on medium-high speed until smooth and fluffy, about 30 seconds. Mix in the egg and vanilla extract.

Reduce mixer speed and gradually add the dry ingredients, mixing until just barely combined. Use your hands to finish bringing the dough together until a soft, pliable dough forms. Do not knead.

With a kitchen scale,* divide the dough into 6 portions, measuring approximately: 160 grams; 140 grams; 120 grams; 100 grams; 80 grams; and 50 grams.

With gloved hands, work 1-2 drops of gel food coloring into each portion. When finished, you should have: Red (160 grams); orange (140 grams); yellow (120 grams); green (100 grams); blue (80 grams); and purple (50 grams).

On a silicone mat, roll the purple dough into a rope, about 8 inches long.

Place the blue dough between 2 sheets of parchment. With a rolling pin, roll the blue dough into a rectangle, approximately 8 x 2 inches, and about 1/8-inch thick. Wrap it around the purple tube. Place tube on prepared tray and freeze while you roll the green dough.

Place the green dough between 2 sheets of parchment. With a rolling pin, roll the green dough into a rectangle, approximately 8 x 3 inches, and about 1/4-inch thick. Wrap it around the blue tube. Place on prepared tray and freeze while you roll the yellow dough.

Place the yellow dough between 2 sheets of parchment. With a rolling pin, roll the yellow dough into a rectangle, approximately 8 x 4 inches, and about 1/4-inch thick. Wrap it around the green tube. Place on prepared tray and freeze while you roll the orange dough.

Place the orange dough between 2 sheets of parchment. With a rolling pin, roll the orange dough into a rectangle, approximately 8 x 5 inches, and about 1/4-inch thick. Wrap it around the yellow tube. Place on prepared tray and freeze while you roll the red dough.

Place the red dough between 2 sheets of parchment. With a rolling pin, roll the red dough into a rectangle, approximately 8 x 7 inches, and about 1/4-inch thick. Wrap it around the orange tube.

Wrap log in clingfilm/plastic wrap and refrigerate until very firm, at least 3 hours, or overnight.

Preheat oven to 350° F / 176° F. Line a half sheet rimmed baking tray, or 2-3 smaller trays, with silicone baking mats.

Remove plastic wrap. With a very sharp chef’s knife, trim edges, then slice log into 1/4-inch thick rounds.

Slice each round in half to create the classic “rainbow” shape.

Arrange slices on prepared tray. Freeze 10 minutes. (If baking in batches, refrigerate remaining slices until ready to bake.)

Bake in preheated oven for 11-13 minutes, or until set, with a matte appearance.

Working quickly, use a metal bench scraper to reshape the bottom of each rainbow, if necessary.

Cool 5 minutes on tray, then transfer to a wire rack to cool completely.

Store rainbow cookies in an airtight container at room temperature for up to 5 days, or freeze for up to 3 months.