Pre-heat the oven to 350ºF. If your oven runs hot, or if it's convection or gas, use 325ºF instead. Grease with spray oil, and line a 8×8” square pan with parchment paper. Don’t use a glass pan.

Mix the butter and chocolate in a large bowl. Place the bowl in the microwave or over a double boiler to melt them together. Microwave for 15 to 30 second increments, stirring in between until the chocolate has melted completely with the butter. If using a double boiler, make sure the bowl is heat-proof, and place it over a pan with simmering water, and whisk the mixture until the chocolate and butter have melted together.

Add the granulated sugar to the bowl and whisk until combined

Add the eggs, one at a time, whisking until completely incorporated before adding the next egg.

Along with the final egg you can add the vanilla.

Whisk to combine.

Sift the flour and the cocoa powder together and add it to the bowl. Also add the Oreo crumbs and salt to the bowl. Stir with a spatula to combine.

Add the chopped Oreos and chopped chocolate, or chocolate chips. Stir to combine.

Set aside.

Mix the unsalted butter and white chocolate in a large bowl. Place the bowl in the microwave or over a double boiler to melt them together.

Add the granulated sugar to the bowl with the melted white chocolate and butter. Whisk to combine.

Add the egg and the vanilla to the batter and mix.

Add the flour and salt. Stir with a spatula to combine.

Add the chopped oreos and mix.

Pour half of the chocolate batter on the bottom of the prepared pan. Use a spatula to smooth it out.

Pour the white brownie batter on top, and smooth it out with a spatula to distribute the batter on an even layer.

Top with the remaining chocolate brownie batter. It helps to use a scoop to evenly distribute the batter on top of the white layer. Then you can use a spatula to gently press down on the chocolate batter and spread it out, avoiding mixing the white and the chocolate batters together.

Top the brownies with a few chopped Oreos if desired.

Bake the brownies in the oven for about 40 minutes, until the brownies look set around the edges and on top. And a toothpick inserted in the brownies doesn’t come out with gooey batter stuck on it.

Remove the brownies from the oven.

Let them cool down before slicing and serving.

Store the brownies in an air-tight container, at room temperature for up to 2 days. Or in the fridge, for up to 6 days. Let them come to room temperature by letting them sit on the counter for 20 minutes before serving. You can also freeze the baked brownies for up to 2 months. Thaw in the fridge overnight.