Preheat your oven to 180C / 355F. Spray two 8-inch cake tins with oil spray and line the bottom with baking paper. Set aside.
Add the all-purpose flour, cocoa powder, baking powder, salt and bicarb soda to a large mixing bowl. Use a whisk to combine. Set aside.
To a separate large bowl, add the butter, sugar and vanilla extract. Use an electric hand mixer to beat on high speed until pale, about 3 minutes. Stop the mixer, scrape down the bowl and turn back on high speed. Add the eggs one at a time while the mixer is on.
Stop the mixer, scrape down the bowl and add half the dry ingredients and half the milk. Mix on low speed until no dry ingredients are showing. Scrape down the bowl and add the remaining dry ingredients and milk. Mix until batter is uniform. Add 1 cup of chopped Oreos and fold through using a spatula.
Split the mixture between the cake tins and bake for 40 minutes or until a toothpick inserted into the center comes out clean. Once baked, allow to cool completely. Once baked and cooled, use a serrated knife to carefully trim the tops off while still in the cake tin.
Add the butter and vanilla extract into a large mixing bowl and beat on high speed until pale and creamy, about 5 minutes. Stop the mixer and run the powdered sugar through a sieve. Mix on low speed until no dry ingredients are showing, then pump up to high speed and beat for 5 minutes. Add the hot milk (see notes) and beat for an extra 60 seconds. Add the crushed Oreos and fold through until well combined.
Add the frosting to a piping bag and pipe a little on your serving plate. Use a spoon or offset spatula to spread around. Add the first layer of cake and pipe a ring of frosting around the top. Fill in with more frosting and spread around evenly. Sprinkle with 1/2 cup of chopped Oreos then add the next layer of cake and cover in frosting. Use the video as a guide to smoothen the frosting as neatly as you can. Add chopped Oreos around the bottom, drizzle chocolate sauce around the top. Fit the end of a piping bag with an open star tip and pipe swirls around the cake. Finish with a wedge of chopped Oreos.
StorageThis cake can be stored in an airtight container for up to three days.CookiesYou don’t have to use Oreos in this recipe. Any sandwich cookie will work. And leave the cream in! No need to scoop it out.Hot milkI heated my milk up in the microwave for 20 seconds. Adding hot milk into the frosting will melt some of it and as that melted frosting mixes into the rest of the frosting it helps makes it smooth and glossy. I would not recommend this technique during warm weather. The frosting will already be susceptible to melting easily.