Line an 8x8-inch square baking pan with parchment paper, and set aside.
Melt the 10 tablespoons of butter in a saucepan with 1/2 cups of sugar, 3/4 cups of unsweetened cocoa powder, and 1/4 teaspoon of salt and stir until smooth.
Remove from heat and allow to cool for a few minutes, then transfer to a large bowl.
Whisk in the two large eggs, one at a time.
Then, stir in vanilla powder or vanilla extract until combined.
Fold in the 1/2 cups of all-purpose flour until fully combined.
Then, gently fold in the chocolate chunks. Make sure that they don't all melt.
Pour half of the batter into the 8 x 8 parchment paper lined pan. Spread the batter out evenly.
Place the 18 whole Oreos evenly over the batter.
Pour the remainder of the remaining batter.
Top with the 8 chopped Oreos.
Bake for 30 minutes.
Check for doneness by inserting a toothpick into the center to see if comes out clean with a few crumbs to oozy wetness. If the brownies are not done, put in the oven for 2-minute increments until the toothpick comes out mostly clean.
Allow the Oreo Brownies to cool in the pan on a wire rack before cutting the brownies.
These Oreo Cookie Brownies will stay fresh when stored in an airtight container, at room temperature or in the refrigerator for up to 1 week. You can also freeze them for up to 3 months.