Prepare the Cake: Preheat your oven to 200C (400F). Cream the margarine and sugar together until light and fluffy. Add the eggs one at a time and mix until fully incorporated. Add the sour cream and combine. Gradually add the flour and baking powder, mixing until you have a smooth cake batter. Pour the batter onto a baking sheet lined with parchment paper and spread it evenly.
Bake: Bake until the cake is golden and cooked through about 10-15 minutes, depending on your oven and the thickness of the cake. Allow the cake to cool completely.
Make the Buttercream: While the cake is cooling, prepare the cream. Beat the softened unsalted butter until smooth and creamy. Gradually add the sweetened condensed milk and continue mixing until you have a smooth, rich buttercream.
Cut the Cakes: Once the cake has cooled, use a small round glass or cookie cutter to cut out little circles. Keep all of the leftover cake scraps- you'll need them for the crumb coating.
Assemble the Little Cakes: Spread a layer of the buttercream onto one cake circle on the tops and sides, then crumble the reserved cake scraps into fine crumbs and roll or gently press each little cake into the crumbs until completely covered.
