PREPARE PAN: Line a large cookie sheet (18x13 inches) with parchment paper.
MAKE BASE DOUGH: In a stand mixer fitted with the whisk attachment, beat 2 cups unsalted butter, softened, 1 cup powdered sugar, 1/4 cup packed light brown sugar, and 1/2 teaspoon kosher salt on high speed until smooth and slightly fluffy, about 2 minutes.
ADD FLOUR: With the mixer on low speed, add the 4 cups all-purpose flour gradually. Mix just until a soft dough forms and pulls from the sides of the bowl.
DIVIDE DOUGH: Divide the dough in half (about 20 ounces each). Set aside one half and leave the other in the mixer bowl.
MAKE CHOCOLATE DOUGH: Add the melted 1/2 cup semi-sweet chocolate chips and 1 tablespoon unsweetened cocoa powder to the dough in the bowl. Beat until it is evenly combined and smooth.
CHILL DOUGH: Wrap both dough halves tightly in plastic wrap. Chill in the refrigerator for 10 minutes.
ROLL DOUGH: Remove one dough half and roll it between two large sheets of parchment paper into a 12x18-inch rectangle, about ¼ inch thick. Repeat with the other dough half.
LAYER & TRIM: Stack the chocolate dough on top of the plain dough. Trim edges to form even sides (save scraps for marble cookies).
ROLL INTO LOG: Starting along the long edge, roll up the dough tightly into a log, pressing gently to prevent gaps. Wrap the log in plastic wrap and chill for 1 hour to firm.
SLICE & BAKE: Preheat the oven to 325°F. Unwrap the chilled dough log and slice into ½-inch rounds. Arrange cookies 1½ inches apart on the prepared baking sheet. Bake for 11 to 13 minutes, until the edges are lightly golden.
COOL: Let cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
Notes
Make-Ahead: The dough can be prepared up to 2 days ahead. Keep wrapped and chilled until ready to bake.Storage: Store cooled cookies in an airtight container at room temperature for up to 5 days.Freezing: Freeze baked cookies or unbaked dough logs for up to 3 months. Thaw at room temperature before baking or serving.Chocolate Tip: Measure ½ cup of chocolate chips before melting. Melt in the microwave at 50% power, stirring every 30 seconds until smooth.Leftovers: Don’t toss dough trimmings! Lightly knead scraps together and bake into marbled chocolate butter cookies.