Put the margarine, oil, caster sugar, milk, cocoa, eggs and self-raising flour in a large mixing bowl.
Whisk, starting slowly and then speeding up until you have a light, smooth batter.
Grease a 12-hole mini cake tray (or muffin tray) and spoon the batter evenly between them. I find the easiest way to do this is with a large ice cream scoop. Don't forget to level off.
Bake for 20-25 minutes until springy to the touch. If you take them out too soon, they'll sink.
Transfer the cakes to a wire cooling rack and allow to cool completely.
Make the ganache frosting
Melt the chocolate in a heatproof bowl in the microwave using 30 second bursts, stirring between each burst.
Add the double cream and stir until thick and glossy.
Transfer to a piping bag fitted with an open star nozzle.
Assemble the mini layer cakes
Use a sharp serrated knife to cut each mini cake into two halves. If they are particularly domed on top, you can also trim off the peaks a little.
Pipe flat roses onto the bottom halves of each mini cake. To do this, start from the centre and pipe in a widening spiral.
Place the cake tops on top of the frosting to create little layer cakes.
Pipe another rose on top of each mini cake.
Serve immediately or if you'd like them a little firmer, pop in the fridge for 30 minutes.