Preheat oven to 350 degrees F. Line a 8-inch-by-8-inch square baking pan with parchment paper so that it overlaps on each side. Spray the parchment paper with non-stick baking spray or coconut oil.

To make the brownie batter, add melted butter, oil, sugar, eggs, and vanilla extract to a large bowl. Whisk until pale in color, about 1 minute. Add flour, cocoa powder, and salt to the bowl. Using a rubber spatula, fold the dry ingredients into the wet ingredients until combined and the batter is smooth.

Transfer the brownie batter to the prepared baking pan, spreading the batter evenly into the bottom of the pan.

To make the cookie dough, add the butter, granulated sugar, brown sugar, and vanilla extract to the bowl of a stand mixer fitted with the paddle attachment (or to a large bowl using a handheld mixer). Beat until light and fluffy, about 1 minute. Scrape down the sides of the bowl and the egg, beating until just combined. Scrape down the sides of the bowl.

Add the flour, baking soda, and salt to the bowl. Beat on low until just combined. Fold in the chocolate chunks and slivered almonds.

Using a 1-tablespoon scoop, drop the batter directly on top of the brownie batter until the brownie batter is covered. Gently press the down on the cookie dough so that it sinks into the brownie batter slightly. Bake for 30-35 minutes, or until the cookie dough is golden brown in color.

Cool in the pan completely. Remove from the pan and cut into squares. Store in an airtight container for up to 5 days.