Line a muffin tray with paper cups and preheat the oven to 180°C.
Combine the sugar and the margarine, which needs to be at room temperature. Cream with an electric mixer for 3-4 minutes, until light and fluffy. Add the eggs, one at a time, and keep mixing until all is nicely combined.
Mix the flour and baking powder, and add to margarine mixture. Mix at low speed until combined. Pour in the cream and just mix until it's all homogeneous. Divide batter into 8 paper cups and bake in the preheated oven for 20 minutes at 180°C.
Combine the egg whites and powdered sugar and whisk over a double boiler until the mixture feels smooth to the touch.
Transfer the mixture to a bowl and mix at medium-high speed until the mixture has cooled down to room temperature (this can take 5-6 minutes).
When the mixture feels at room temperature, slow the mixer to medium-low and add the butter, little by little. The butter must be at room temperature. When all the butter has been incorporated, add the peppermint extract. 2-3 drops should be enough, but adjust to taste.
When the cupcakes have completely cooled down, pipe the peppermint buttercream over each cupcake. Top with some crushed candy cane for extra colour and flavour.