Preheat the oven to 160ºC (320ºF) and line a 12-cup cupcake pan with liners.

In a large mixing bowl, sift together flour, baking powder, baking soda, salt, and cinnamon. Mix in granulated and brown sugar until combined.

In a separate bowl, whisk eggs, sour cream, vegetable oil, and vanilla extract until blended.

Pour the wet ingredients into the dry mixture and stir until just combined.

Divide the batter among the cupcake liners, filling each about ¾ full.

Bake for 20-22 minutes, checking doneness with a cake tester.

Cool the cupcakes in the pan for 5 minutes, then transfer to a wire rack to cool completely.

For the buttercream, beat softened butter until fluffy, gradually adding powdered sugar, milk, and vanilla until creamy.

Divide the buttercream and mix one half with cinnamon for a spiced version.

Decorate the cooled cupcakes using a piping bag with a swirl of both buttercreams.