Preheat and Prep: Preheat oven to 350°F (175°C). Line a 9x9-inch baking pan with parchment paper and lightly spray with non-stick baking spray.

Mix Dry Ingredients: Sift together 3/4 cup flour, 1/2 teaspoon salt, and 1/2 cup cocoa powder in a large bowl. Set aside.

Prepare Butter Mixture: In a medium saucepan, melt 3/4 cup butter, 3 tablespoons oil, and 1/2 cup sugar over medium heat, stirring frequently. Remove from heat.

Combine Eggs and Sugar: In a large mixing bowl, whisk together 2 eggs, 1 egg yolk, and the remaining 1 cup sugar until well combined. Stir in 2 teaspoons vanilla extract.

Incorporate Butter Mixture: Gradually pour the warm butter mixture into the egg mixture, whisking constantly until fully combined.

Add Dry Ingredients: Slowly stir in the dry ingredients with a rubber spatula until just combined.

Fold in Add-ins: Fold in 3/4 cup chocolate chips and half of the roughly chopped 14 Oreo cookies. Do not overmix.

Bake: Pour batter into the prepared pan and smooth the top. Press the remaining Oreo cookies on top. Bake for 30 minutes, or until the edges are firm and the top is shiny and slightly cracked. Cool on a wire rack for at least 2 hours before slicing.