Preheat your oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners. Set aside.
In a medium bowl, whisk together the flour, cinnamon, and baking powder until evenly combined. This ensures no clumps of spice remain.
In a large bowl, beat the room temperature butter and brown sugar using an electric mixer on medium-high speed for 3 full minutes. The mixture should be pale and fluffy—this step is key for airy cupcakes.
Add the eggs one at a time, beating well after each (about 30 seconds each). Then mix in the vanilla extract.
Reduce mixer speed to low. Alternate adding the milk and flour mixture in three additions, beginning and ending with dry. Mix only until just combined—do not overmix.
Fill each cupcake liner 2/3 full (about 3 tablespoons of batter each). Bake for 18–20 minutes or until a toothpick inserted in the center comes out clean or with a few moist crumbs. The tops should gently spring back when pressed. Cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
Once cooled, use a cupcake corer or paring knife to remove a small cylinder from the center of each cupcake, about 1 inch deep. Warm the caramel sauce slightly until pourable (not hot), then spoon about 2 teaspoons into each cavity.
In a large bowl, beat butter on medium speed until creamy (about 2 minutes). Gradually beat in powdered sugar, half a cup at a time, scraping down the bowl as needed. Add caramel sauce and cinnamon, then beat for an additional 3 minutes until fluffy and pipeable.
Transfer frosting into a piping bag with a large star tip. Pipe a generous swirl onto each cupcake. Press crushed Cinnamon Toast Crunch gently onto the sides of the frosting immediately after piping. Finish with a caramel drizzle and a pinch of sea salt, if desired.