Preheat the oven to 180℃/160℃ fan (350℉/320℉).
Add the butter and both sugars to a large mixing bowl. Beat with a electric hand mixer until light and creamy. Add the egg and vanilla and beat on a medium speed for a further minute.
Tip in the flour, bicarbonate of soda, cocoa powder, and salt. Stir through with a rubber spatula until just combined.
Tip in the chocolate chunks and crushed Oreos, stir until spread evenly through the dough.
Line two large baking trays with baking paper.
Using an ice cream scoop, scoop a spoonful of cookie dough onto the baking paper. Make a dip in the dough using two fingers and press in a whole Oreo. Top with another half scoop of cookie dough. Press the dough around the Oreo so it's completely covered.
Repeat until you have 8 evenly sized cookies. Space well apart on the baking trays and bake for 12 minutes. Leave to cool for 10 minutes on the trays before moving to a wire rack to cool completely.
