Adjust oven rack to the middle position and preheat oven to 350°. Line a baking sheep pan with parchment paper.
In a medium bowl, combine 2 ½ cups flour, 1 Tablespoon cornstarch, ½ teaspoon baking soda, and ¼ teaspoon salt. Set aside.
In the bowl of a stand mixer fitted with a paddle, beat 1 cup butter on medium speed until creamy, about 1 minute. Add 1 ¼ cups sugar and beat on medium speed until light and fluffy, about 2 minutes. Add 1 egg, 1 yolk, and 2 teaspoons vanilla, and beat on medium speed until combined. Add the flour mixture and beat on low speed until just combined.
Dump the dough out onto counter or work surface and divide it into three equal portions. Put one-third of the dough into the mixer and add ½ teaspoon of lemon zest. Then add 2 ½ Tablespoons sweet potato powder, mix on medium until fully incorporated and the dough is a lovely purple throughout. Set aside.
Add another third portion of the dough into the mixer and add 2 Tablespoons cocoa powder. Mix on medium until the dough turns absorbs all the cocoa powder and turns a nice dark chocolate (or black if using black cocoa powder) color. Remove from the bowl.
Pinch small portions (about ½ oz or ½ Tablespoon size) of the vanilla dough until all the dough has been portioned. Repeat with the purple and then the chocolate dough. You will now have several small balls of all the dough ready to be combined.