In a medium bowl, whisk together the all-purpose flour, baking powder, and salt. Set this mixture aside for later use.
In a large mixing bowl or the bowl of a stand mixer, beat the softened butter and granulated sugar together until the mixture is light and fluffy. This should take about 2 to 3 minutes.
Add the egg and vanilla extract to the butter mixture. Beat well until fully combined, scraping down the sides of the bowl as needed.
Gradually add the dry flour mixture to the wet ingredients. Mix on low speed just until the flour disappears. Do not overmix, as this can make the cookies tough.
Divide the dough into two equal portions. Leave one portion plain (vanilla). To the second portion, add the cocoa powder and milk (or food coloring) and mix until the color is uniform.
Roll each piece of dough between two sheets of parchment paper into a rectangle about 1/4 inch thick. Aim for roughly 9×12 inches for both rectangles.
Carefully place one dough rectangle on top of the other. Starting from the long edge, roll the dough tightly into a log. If desired, roll the log in festive sprinkles at this stage.
Wrap the log tightly in plastic wrap and refrigerate for at least 2 hours. This step is crucial to ensure the cookies hold their shape when sliced.
Preheat your oven to 350°F (175°C). Use a sharp knife to slice the log into 1/2-inch thick rounds. Place them on a parchment-lined baking sheet about 2 inches apart. Bake for 10-12 minutes or until the edges are just set.