Begin by lining an 8 x 8-inch metal baking pan with parchment paper or foil, making sure to leave enough overhang on the sides to lift the filling out easily later.

Transfer the cookie butter to a microwave-safe glass measuring cup and warm it for 20 to 30 seconds, just until it becomes soft and easy to pour. Be careful not to let it get too hot. Pour the melted cookie butter into the prepared pan, spreading it into a smooth, even layer.

Place the pan in the freezer for about 1 hour, or until the cookie butter has completely firmed up. Once solid, lift it out using the parchment paper and return the cookie butter sheet to the freezer while you prepare the brownie batter. Keeping it frozen will make it easier to handle when assembling the brownies.

Preheat your oven to 325°F. Line an 8 x 8-inch baking pan with a fresh piece of parchment paper or foil, then lightly coat it with nonstick cooking spray.

Place the butter in a large microwave-safe bowl and heat until completely melted. Add both sugars and whisk until thoroughly combined. Set the mixture aside for a few minutes to cool until just warm.

Add the eggs and egg yolk, whisking vigorously for about 1 minute. Mix in the vanilla extract, oil, and cocoa powder until the mixture is smooth. Using a rubber spatula, gently stir in the flour, cornstarch, salt, espresso powder, and cinnamon. Mix only until all the dry ingredients are incorporated.

Spread half of the brownie batter evenly across the bottom of the prepared pan. Carefully remove the frozen cookie butter layer from the freezer and place it over the brownie batter. Cover with the remaining brownie mixture, making sure the cookie butter is completely enclosed. Finish by scattering the chopped cookies over the top, pressing them gently into the batter so they stay in place.

Bake for 32 to 35 minutes, or until the brownies are set. Keep a close eye on them, as overbaking can take away that lovely fudgy texture.

Remove the pan from the oven and allow the brownies to cool completely. This resting time is important because it helps them firm up, making them much easier to cut into neat squares.

Store any leftovers in an airtight container at room temperature for up to 3 days. These brownies are delicious in different ways depending on how you serve them: warm, they have a wonderfully gooey texture; at room temperature, they become pleasantly chewy; and when chilled, they develop an especially rich, fudgy consistency.

Cocoa Powder: Dutch-process cocoa powder is my first choice for these brownies because of its deep, rich chocolate flavor. However, natural unsweetened cocoa powder works just as well. You can substitute it in equal amounts.

Baking Pan: A metal 8 x 8-inch pan is strongly recommended for the best baking results. Glass and ceramic pans are not ideal for this recipe. If you only have one of these, allow an additional 5 to 10 minutes of baking time, checking carefully for doneness.