To make the chocolate cake layers:

1. In a bowl, whisk together hot water and cocoa powder until the cocoa has dissolved. Set aside to come to room temperature.
2. In a mixer with a beater attachment, mix butter with sugar on medium speed until pale and creamy, about two minutes. Scrape down to the bottom of the bowl once during mixing to make sure no butter is stuck at the bottom.
3. Add eggs one at a time, mixing on the lowest speed after each addition until incorporated, about 30 seconds.
4. Add vanilla and mix on lowest speed for about 30 seconds to combine.
5. In a bowl sift flour, baking soda, baking powder and salt. Add a third of the flour mixture to the mixing bowl and mix on lowest speed to incorporate.
6. Add half of the cocoa mixture to the mixing bowl and mix on lowest speed to incorporate. Repeat with another third of the flour mixture, then the remaining cocoa powder mixture, and then the remaining flour mixture. Mix just until the ingredients are combined.
7. Grease the inside (base and sides) of three 8" pans with non-stick spray, butter or oil. Divide batter between the cake pans.
8. Bake at 175ºC or 350ºF for 38 minutes, or until a toothpick inserted into the middle of the cake comes out clean.
9. Let cakes cool in their pans for 10 minutes. Use a spatula to loosen cake from the edges of the cake pan and then turn onto a wire cooling rack. Leave to cool completely.

To make the Oreo buttercream filling and frosting:

1. Sift the sugar to remove any lumps and check that the butter is at room temperature - you should be able to slice through it easily with a spatula.
2. In a mixer with a beater (paddle) attachment, mix butter for a few seconds until smooth. Add a quarterof the powdered sugar and salt and mix on the lowest speed until incorporated, about one minute.
3. Scrape down to the bottom of the mixing bowl with a spatula to loosen any butter and sugar and add the next quarter of powdered sugar and mix for another minute on low.
4. Scrape down to the bottom of the bowl, add another quarter of the powdered sugar, mix for one minute on low, scrape, and add the final quarter of the powdered sugar. Mix for one more minute on low, adding the vanilla once everything is mixed together and continuing to mix until it's incorporated.
5. Add the milk or cream and mix for about 30 seconds until it's combined with the buttercream. Check the consistency and add more milk or cream a tablespoon at a time until the buttercream is smooth and easily stirred.
6. Crush 4 Oreos in a food processor or you can put them in a sandwich bag and bang them with a rolling pin or something heavy to crush them. Pour them into the buttercream and fold them in, stirring as little as possible to avoid making grey buttercream. 

To put the cake together:

1. Spread some buttercream onto the middle of a cake board to attach your first cake layer. Spread on the Oreo buttercream as filling, going right up the to the edges. Spin the cake to smooth the buttercream, holding your offset spatula or palette knife at a 45 degree angle. 
2. Chill the cake in the fridge for 30 minutes and then spread Oreo buttercream to frost it with a crumb coat. Scrape around the cake with a cake comb to get the frosting fairly smooth and swipe sideways with your offset spatula or palette knife to level the top edge. Chill for another 15- 30 minutes in the fridge to set this crumb coat.
3. Spread Oreo buttercream to cover the cake with a final layer of frosting. Smooth with a cake comb.
4. Push 12 Oreos into the top of the cake to stand upright around the edge, spacing them evenly apart from each other. I arrange 4 cookies at 12 o'clock, 6 o'clock, 3 o'clock and 9 o'clock and then add 2 more cookies in between each of those.

5. Fit a piping bag with a star shaped piping tip like a 1M and spoon the leftover Oreo buttercream into the piping bag. Pipe swirls between the cookies, going out to the edge of the cake and all the way up to the cookies on each side, and stop when you get to the top of the cookies.

6. You can keep this cake in the fridge for 3 days. Take it out 2-4 hours before you serve it so that the cake and buttercream come to room temperature and that’s when they taste the best!