Pre-heat oven to 350 degrees and line a cupcake tin with 12 papers.
In a medium bowl, whisk together flour, baking powder, cinnamon, and salt.
In the bowl of a stand mixer or in a large bowl with a hand mixer, cream together butter and sugar on medium-high speed until light and fluffy, about 3 minutes.
Add eggs in one at a time, mixing well after each addition. Mix in vanilla extract.
With mixer on low speed, beat in dry ingredients in 3 additions, alternating with the milk. Mix in each addition until just incorporated, taking care not to over-mix.
Fill cupcake liners about ⅓ of the way with batter. Evenly divide cinnamon swirl mixture across batter in each cupcake paper. Top with remaining batter. Cupcake papers should be filled about ¾ of the way.
Bake cupcakes for 17-20 minutes, until edges are golden and tester comes out clean. Let cupcakes cool in pan for a few minutes before removing to wire racks to cool completely.
When cupcakes are cooled, frost them (I used a Wilton 1M pastry tip) and top them with crumb topping.