Whisk the flour, baking powder, and salt together in a medium bowl. Set aside.

In a large bowl, beat the butter and granulated sugar together until creamed, about 2 minutes. Add the egg, egg yolk, and vanilla extract and beat until combined, about 1 minute.

Add the dry ingredients to the wet ingredients and mix until combined. The dough will be thick and sticky.

Divide the dough into 3 equal portions, about 10 ounces each. One portion is your vanilla dough. Set aside.

For the strawberry dough, add all the strawberry dough ingredients to one portion of the dough and beat until combined. Add food coloring until the desired pink color is reached. Remove from the bowl.

For the chocolate dough, add all the chocolate dough ingredients to the remaining portion of dough and beat until combined.

Line a 9?5-inch loaf pan with aluminum foil or parchment paper. Layer the dough in the following order: strawberry dough on the bottom, vanilla dough in the middle, and chocolate dough on top. Press each layer firmly and evenly into the pan.

Cover the loaf pan tightly and chill the dough in the refrigerator for at least 3 hours or in the freezer for 1 hour.

Preheat the oven to 350?F (177?C) and line 2 baking sheets with parchment paper or silicone baking mats.

Lift the dough out of the pan and slice it in half lengthwise to create two 2.5 x 9-inch rectangles. Slice each rectangle into 16 cookies.

Arrange cookies 2-3 inches apart on the baking sheets. Optionally brush each lightly with egg wash and sprinkle with coarse sugar. Bake for 11-13 minutes or until lightly browned around the edges.

Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.