In a large bowl, cream together the softened butter, granulated sugar, and brown sugar using a hand mixer or stand mixer on medium speed until light and fluffy, about 2–3 minutes.

Add the egg and vanilla extract to the butter-sugar mixture and beat until fully incorporated, about 30 seconds.

In a separate bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking soda, and kosher salt until evenly combined.

Gradually add the dry ingredients to the wet mixture, mixing on low speed until just combined. Do not overmix.

Fold in the rolled oats and semi-sweet chocolate chips with a spatula until evenly distributed throughout the dough.

Transfer the cookie dough to the refrigerator to chill for 15–20 minutes. Chilled dough will be easier to shape and will hold structure when layered with croissant dough.

Croissant Layer (Flaky Butter Layer)

If using store-bought croissant dough sheet: unroll the sheet on a lightly floured surface. If using puff pastry, thaw according to package directions and roll to a rectangle roughly 12 x 9 inches.

On a lightly floured surface, roll the chilled cookie dough into a rectangle roughly the same size as the croissant/puff pastry sheet (about 10–11 x 8 inches). It will be thick; aim for about 1/4–1/3 inch thickness.

Place the cookie dough rectangle on top of the croissant/puff pastry sheet, aligning edges as best as possible.

Starting from the short edge, gently roll the pastry and cookie dough together into a tight log, sealing the seam by pinching the pastry together. If the dough sticks, chill the assembled log for 10 minutes to firm up.

Slice the log into 12 even rounds (about 1 inch thick each) using a sharp serrated knife or dental floss for a clean cut. Place the slices, cut-side up, on the prepared baking sheet spaced 2 inches apart to allow for spreading.

In a small bowl, beat the remaining egg with 1 teaspoon water to make an egg wash. Brush the tops and exposed croissant edges of each crookie with the egg wash to encourage browning and flakiness.

Bake in the preheated 350°F (175°C) oven for 12–16 minutes, or until the croissant layers are golden and cookie centers are set but still slightly soft. Rotate the pan halfway through baking for even color.

Remove from oven and allow crookies to cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely. Cooling helps the cookie set while preserving a flaky croissant exterior.