Spray a 7-inch microwave-safe silicone cake mold with cooking oil and line the bottom with a circle of parchment paper. 

Separate the sandwich cookies and scrape the cream filling into a bowl, then set aside. Put the separated cookies into a food processor and process until finely crushed. Alternatively, put them into a plastic bag and crush with a rolling pin. Tip into a large mixing bowl, reserving about 1 tablespoon of the crumbs for topping, if decorating the cake.  

Add the baking powder to the large bowl of crumbs and mix together with a balloon whisk. Pour in the milk, then add the cookie cream filling to the bowl and mix together until smooth.  

Pour into the prepared mold and microwave on high for 6 to 7 minutes until a toothpick inserted into the middle comes out clean. (If you prefer, you can bake the cake in an oven preheated to 350°F for 15 minutes.) Leave to cool in the mold, then invert the cake onto a cooling rack and peel off the lining paper.  

The cake can be topped with the ganache below and/or reserved cookie crumbs or cut into slices and served as is. Enjoy! 

To make the ganache (if using), heat the cream in a glass jug or microwave-safe bowl in the microwave for about 1 minute until hot, but not boiling or bubbling. (It may need longer, depending on your microwave.) 

Put the chocolate into a medium heatproof bowl, then pour over the hot cream and gently stir until smooth and combined. Pour over the cooled cake, letting it drip over the edges, then sprinkle with the reserved cookie crumbs, if decorating the cake.  

Store in an airtight container in the refrigerator for up to 3 days.