How to Make It
Preheat the oven to 350º. In the bowl of a stand mixer using the paddle attachment, beat the butter and brown sugar on high speed until lightened and fluffy, about 3 minutes. Add the eggs and the vanilla and beat on medium-low to incorporate.
In a medium bowl, add the flour, baking powder, salt and cinnamon; whisk to combine. Alternate adding the flour mixture (in three batches) and the milk (in two batches), starting and ending with the flour mixture; mix on low speed after each addition until just combined. Do not overmix.
Line 24 cupcake cups with paper liners. Using a cookie or ice cream scoop, divide the batter among the cupcake liners; wipe away any batter off the tops of the cups. Bake, rotating the pans once halfway through baking, until the cupcakes have risen and a toothpick or cake tester inserted in the center comes out clean, about 18 minutes. Remove and let cool completely. Can be stored overnight wrapped in plastic wrap before frosting.
Make the frosting: In a medium saucepan, cook the butter, brown sugar and milk over medium-low heat, stirring occasionally to incorporate the milk, until the butter is melted and the sugar is dissolved, about 3 minutes. Remove from the heat and transfer to the bowl of a standing mixer. Add the powdered sugar and beat on high until smooth.
Frost the cupcakes while the frosting is still warm for the smoothest results. Spread each cupcake with a thin layer of frosting, then garnish with 2 to 3 pieces caramel corn and a pinch of flaky sea salt.