Using an electric stand mixer with a paddle attachment (or heavy-duty hand mixer) on medium speed, cream together the butter and sugar until light and fluffy. This will take about 3 minutes with the paddle attachment of a stand mixer.

Add the eggs and vanilla, and continue mixing.

Add the flour, baking powder, and salt, and mix until incorporated. The dough will be crumbly but will hold together when pressed.

Divide the dough into 2 equal portions, and place one half back into the mixing bowl. Add 1/2 tablespoon of the milk to the portion in the mixing bow, and mix until combined. Turn the dough onto the counter, and shape into a thick rectangle. Wrap tightly in plastic wrap.

Place the second half of the dough in the mixing bowl, and add the remaining 1 tablespoon milk, cocoa powder, and espresso powder (if using). Mix until combined. Shape the chocolate dough as you did the vanilla dough. Wrap tightly in plastic wrap. Refrigerate both doughs for 30 minutes.

Once the doughs have chilled, place each dough rectangle on a separate piece of parchment paper approximately 9x13 inches in size. Lightly flour a rolling pin, and then roll out each dough to a rectangle 1/4-inch thick. Use the parchment sheets as a guide to try to keep the two doughs roughly the same size and shape.

Gently place one dough on top of the other, carefully lining up the edges as best you can. Work out any air bubbles.

Starting on the long side of the layered doughs, begin tightly rolling the dough into a log. When done rolling, your cookie log should be about 3 inches in diameter. Chill for 30 minutes.

Use a sharp knife to slice the chilled cookie dough log into rounds. Place the sliced cookies on prepared baking sheet, leaving about 2 inches between each cookie.

Bake one pan at a time for 15 minutes, or until the edges begin to turn golden brown. (Keep any remaining dough in the refrigerator between batches.) Repeat with the remaining cookies.

Allow the cookies to cool to room temperature before dipping in chocolate and decorating.