Preheat your oven to 350°F (175°C). Line a 12-cup cupcake tin with paper liners and lightly spray the liners with non-stick spray.
Mix Dry Ingredients:
In a large bowl, combine flour, baking powder, baking soda, salt, and cinnamon. Set aside.
Combine Wet Ingredients:
In a small bowl, mix together the milk and sour cream; set aside.
Cream Butter and Sugars:
In a large bowl, beat the butter, granulated sugar, and brown sugar until light and fluffy (about 4 minutes).
Add Eggs:
Add the eggs and egg yolk one at a time, mixing well after each addition.
Combine Mixtures:
Reduce mixer speed to low and alternate adding the flour mixture and the milk mixture, starting and ending with the flour. Stir in the hot water until just combined.
Fill and Bake:
Fill the cupcake tins ¾ full and bake for 20-22 minutes, or until a toothpick comes out clean. Let them cool in the pan for 10 minutes, then transfer to a cooling rack.
Fill Cupcakes:
Once cooled, carve out a small hole in the center of each cupcake and fill it with about 2 teaspoons of caramel sauce. Replace the carved-out piece.
For the Salted Caramel Frosting:
Make the Frosting:
In a medium saucepan over medium heat, combine butter, cream, brown sugar, and salt. Stir occasionally until the sugar melts and the mixture simmers (about 4 minutes).
Cool and Beat:
Remove from heat and let cool for 2 minutes. Transfer to an electric mixer bowl and add 2 cups of confectioners’ sugar. Beat on medium-low until combined, then add remaining sugar and beat until smooth. Increase speed to medium and beat for 5 minutes.
Chill:
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