Remove the cream part from the Oreos, put it in a bowl, and add the hard cookie parts to a Ziploc bag. Use a rolling pin to crush the cookies into fine crumbs. You can also use a food processor.
Transfer the Oreo crumbs to a large bowl, add baking powder, a bit of salt, cocoa powder, and espresso powder, mix well. Add the milk and stir until fully combined.
Whisk in the cream from the Oreos into the milk mixture. If you still see lumps, you can use a blender to make the cake batter smooth.
Grease a small saucepan with butter or coconut oil.
Transfer the chocolate cake batter to it, cover with a lid, and cook on low heat on the stove for 45-55 minutes. A toothpick inserted in the center of the cake should come out clean or with just a few moist crumbs but no raw batter on it.
Remove the cake from heat, uncover, and let it cool for 20 minutes before inverting it on a plate. Let the cake fully cool.
Chocolate Ganache (optional):
Add chopped chocolate to a medium bowl.
Heat the cream in the microwave for 1 minute, or until hot.
Pour the hot cream on top of the chocolate, let stand for 1 minute, after that stir until fully melted and smooth.
Pour the ganache on top of the cooled cake.
Garnish with berries and refrigerate for 2-4 hours before serving.