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

In a large bowl using a hand mixer or a stand mixer fitted with a paddle attachment, beat the butter and granulated sugar together on medium-high speed until creamed, about 3 minutes. Scrape down the sides and up the bottom of the bowl as needed. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) Add the egg, egg yolk, and vanilla extract and beat on high speed until combined, about 1 minute. Scrape down the sides and up the bottom of the bowl and beat again as needed to combine.

Add the flour mixture to the wet ingredients and mix on low speed until combined. Dough will be thick and sticky. Remove from the mixing bowl—no need to rinse out the mixing bowl.

You should have a little less than 2 lbs. of dough, or somewhere between 800–900g. Divide the dough in half. Note: It’s ok if the portions aren’t exactly half. If one half is slightly smaller, use that one for the chocolate dough.

Place half of the dough back into the mixing bowl. Add the cocoa powder, milk, and espresso powder (if using). Beat on low speed until dough is combined.

The doughs are sticky, so just do your best in this step. Place both doughs on a lightly floured work surface. (Tip: I like to use cocoa powder as my “flour” for handling/rolling the chocolate dough.) With lightly floured hands, carefully form each dough into a 1-inch-thick rectangle block, about 4×5 inches in size. Wrap both dough blocks up in plastic wrap, parchment paper, or aluminum foil. Refrigerate for 60–90 minutes. The *key* is to only refrigerate until the dough is less sticky, but still pliable enough to bend and shape. Chilling any longer will make it difficult to roll together. If you need to chill it for longer than 90 minutes, make sure you let it sit at room temperature for 15–30 minutes before continuing with the next step.

Remove vanilla dough from the refrigerator and unwrap. Keep chocolate dough in the refrigerator. Lightly flour a piece of parchment paper. Note that I like to place the parchment paper on top of a silicone baking mat to prevent it from sliding all over the counter, or you can tape it down. (Do not roll the doughs directly on the counter because you may have a hard time lifting/transferring them.) Using a lightly floured rolling pin, begin rolling out the vanilla dough. Flip the dough every now and then between rolls to ensure it’s not sticking to the parchment. Roll out to an 8×14-inch rectangle, and use your hands to ensure the sides are straight and smooth. The thickness will be between 1/8–1/4 inch. Set the dough aside, and it’s most helpful if you refrigerate the vanilla dough at this time. Pick up the vanilla dough using the parchment paper and place the whole thing in the refrigerator as you roll out the chocolate dough. Lightly dust another piece of parchment paper with flour or cocoa powder. Using a lightly floured (or cocoa powder-ed) rolling pin, roll the chocolate dough out to an 8×14-inch rectangle, or very slightly smaller. Remove vanilla dough from the refrigerator. Carefully peel the chocolate dough off the parchment and do your best to line it up on top of the vanilla dough. Flatten the chocolate dough on top as best you can, gently patting it down to remove any air bubbles between the layers of doughs. Patch any torn chocolate dough pieces back together. And feel free to use a pizza cutter or sharp knife to straighten up any uneven edges.

Starting with a long side, tightly roll the 2 doughs together, making sure there are no gaps between them. You can use the parchment paper under the vanilla dough to assist in rolling. Go slowly, and try to roll it as tightly as possible. You can kind of meld the doughs together with your fingers at the beginning to get the roll started. (You can see me do this in the video tutorial below.) If you notice bare or cracked spots of vanilla dough, which usually happens (don’t worry!), just patch it up with your fingers as you go. Doesn’t have to look perfect. Once you have your log of dough, cut it in half, so you have two 7-inch-long dough logs.

Wrap each dough log in plastic wrap, parchment paper, or aluminum foil and refrigerate for 2 more hours and up to 4 days.

Preheat oven to 350°F (177°C). Line 2 baking sheets with parchment paper or silicone baking mats. Set aside.

Slice each log into 14 1/2-inch-thick cookies.

Arrange cookies about 2 inches apart on the prepared baking sheets—expect these cookies to expand quite a bit in the oven. If the dough got a little soft from working with it, chill the cookies on the baking sheet in the refrigerator for 10 minutes before baking. Bake for 14–15 minutes or until very lightly browned around the edges.

Remove from the oven and allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.

Coarsely chop the chocolate. Melt in a double boiler or the microwave. If using the microwave, stir every 20 seconds until it’s smooth. Dip half of each cooled cookie into the chocolate. Add sprinkles on top of chocolate, if desired. Allow chocolate to completely set at room temperature or in the refrigerator.

Cookies stay fresh covered at room temperature for 1 week. Refrigerate cookies dipped in chocolate after a few days.