• Preheat the oven to 425°F/220°C. Line a muffin tin with paper muffin cups.
• Crush the 4 oreos for the struesel into small pieces. Combine softened butter, granulated sugar, all-purpose flour, and crushed Oreos in a small bowl. Use a fork or your fingers to combine it into coarse crumbs. Set aside. 4 Oreo cookies, 32 g all-purpose flour, 35 g unsalted butter, 25 g granulated sugar, 1/8 teaspoon salt

Muffins

• Chop the remaining oreos into big chunks and set aside. 14 Oreo cookies
• Combine flour, cocoa powder, baking powder, baking soda, and salt in a separate bowl and whisk. Set the flour mixture aside. 250 g all-purpose flour, 30 g unsweetened cocoa powder, 1 1/2 tsp baking powder, 1/2 teaspoon baking soda, 1/2 teaspoon salt
• Add oil, granulated sugar, eggs, milk, sour cream and vanilla in a large bowl. Whisk until combined. 105 g neutral vegetable oil, 2 large eggs, 200 g granulated sugar, 180 g sour cream, 120 g milk, 1 teaspoon vanilla paste
• Add the flour mixture to the wet ingredients and fold them gently with a rubber spatula. Fold the Oreo chunks through the batter.
• Divide the muffin batter evenly in the muffin liners. They will be full. Generously pile the Oreo streusel on top of each muffin.
• Bake the muffins at 425°F/220°C for 5 minutes, then turn the oven down to 350°F/180°C and continue baking for another 18-20 minutes or until the tops of the muffins spring back when lightly pressed in the center and a toothpick inserted comes out clean. 
• Let the muffins sit in the pan for a few minutes, then transfer them to a wire rack to cool completely.