Begin by preheating your oven to 350°F (175°C). This ensures that your bars bake evenly and develop that perfect gooey texture.
Grease an 8x8 inch baking pan with butter or line it with parchment paper for easy removal after baking.
In a medium bowl, whisk together the cocoa powder, baking soda, salt, and flour. Set this mixture aside.
In a large mixing bowl, combine the softened butter, brown sugar, and granulated sugar. Use a hand mixer or stand mixer to cream them together until the mixture is light and fluffy, about 2-3 minutes.
Add the vanilla extract and the large egg to the creamed butter mixture. Mix until fully incorporated.
Gradually add the dry ingredient mixture to the wet ingredients, mixing until just combined. Be careful not to overmix; you want the dough to remain tender.
With a spatula, gently fold in the white chocolate chips and the chopped Oreo cookies. The batter will be thick, but that’s what makes these bars so gooey!
Pour in the sweetened condensed milk and mix until everything is evenly distributed. This is what will give your bars that delicious gooeyness.
Spread the batter evenly into the prepared baking pan. Bake in the preheated oven for 25-30 minutes, or until the edges are set and the center is slightly underbaked (it will continue cooking as it cools).
Once baked, allow the bars to cool in the pan for at least 15 minutes before lifting them out using the edges of the parchment paper (if used). Cut into squares and serve warm or at room temperature.
