Whisk the flour, cocoa powder, sugar, and salt together in a large bowl.
3½ cups all-purpose flour, ⅓ cup unsweetened cocoa powder, 3 tablespoons granulated sugar, ½ teaspoon salt
Cut the cold butter into the flour mixture with your fingers or a pastry cutter until it looks like coarse sand with a few pea-sized bits of butter left.
14 tablespoons unsalted butter
Add the ice water and mix gently just until the dough starts to come together. If it's still too dry, add more cold water a tablespoon at a time, up to 2 extra tablespoons.
½ cup ice-cold water
Shape the dough into a rough ball, cover, and chill for about 10 minutes.
On a lightly floured surface, roll the dough out to about ⅛ inch thick.
Cut into roughly 3x5-inch rectangles and transfer to a parchment-lined tray.
Gather the scraps and reroll up to two more times, aiming for about 12-14 rectangles total.
Chill the rectangles for another 10 minutes while you make the filling.