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. (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 dry ingredients to the wet ingredients and mix on low until combined. Dough will be thick and sticky. Remove from the mixing bowl. No need to rinse out the mixing bowl.

There’s almost 4 cups of dough. Divide in half, a bit less than 2 cups each.

Place half of the dough back into the mixing bowl. Add the mint dough ingredients. I recommend starting with 1 small drop of green food coloring. Beat on low speed until dough is combined. Add another small drop of food coloring if desired. For the pictured cookies, I used 1 drop of leaf green gel food coloring. Remove from the mixing bowl. No need to rinse out the mixing bowl, but feel free to wipe clean.

Place other half of dough back into the mixing bowl. Add all of the chocolate dough ingredients. Beat on low speed until dough is combined.

Place doughs on a lightly floured work surface. It’s sticky, so do your best in this step. With lightly floured hands, carefully form both doughs into a 6 inch long rectangle block, 2 inches tall and 2 inches wide. Wrap both dough blocks up in plastic wrap or place on a plate and cover with aluminum foil. Refrigerate for just 1 hour. 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 form a neat checkerboard pattern. If you need to chill it for longer than 1 hour, make sure you let it sit at room temperature for 15-30 minutes before continuing with the next step.

See photos in the blog post above for visuals of this step. Remove doughs from refrigerator and unwrap. Using a sharp knife, cut each block lengthways into 3 even strips. Then cut each strip into thirds again. You’ll have 9 thin strips from each dough block. My strips are usually a little uneven and messy, so for a neat checkerboard pattern, I always trim off* any rounded edges. Place 1 mint dough strip on your work surface. Line up a chocolate dough strip next to it. Line up a mint dough strip next to that. Top each with a row of alternating flavored dough strips, then top that with another row of alternating dough strips. Repeat with 2nd block, starting with a chocolate dough strip this time. As you’re working, press the strips tightly together to rid any gaps. (This is why it’s important the doughs are still a bit soft and pliable.) If desired to keep the checkerboard pattern neat, trim off* any rounded edges of your blocks.

Wrap each block of dough back up or cover 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 block into 12 1/2-inch thick cookies. For thinner cookies, slice each block into 18 cookies, about 1/3-inch thick. (Bake time will be about 1 minute shorter.) Trim the cookie edges* to make neat squares, if desired.

Arrange cookies 2-3 inches apart on the baking sheets. If desired, brush each lightly with egg wash (see recipe note) and sprinkle with coarse sugar. Bake for 12-14 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.

Cookies stay fresh covered at room temperature for 1 week.