When the cupcakes come out of the oven, each will work as one layer in your mini cakes. Start by placing a small amount of buttercream on a cake stand and set one layer on top.
The buttercream will help keep the cake from moving around. Next, spread a dollop of buttercream frosting onto the cupcake and use an offset spatula to cover the cake.
Top this with another cupcake, placed top-down, and repeat the process. For extra-even layers, you can level the cupcakes once they have totally cooled down.
You can add a small dowel to the middle for support, and for an extra-tall mini cake, add a third cupcake. Finally, smooth out the cake with buttercream and decorate.