Preheat oven to 350°F (175°C).
Line a cupcake pan with paper or foil liners.
In a medium bowl, whisk together flour, baking powder, salt, and cinnamon. Set aside.
In a large bowl, beat softened butter and sugar together on medium speed until light and fluffy, about 1–2 minutes.
Add eggs one at a time, beating well after each addition until fully combined.
Gradually add the dry ingredients to the creamed mixture.
Stir in milk and vanilla extract until smooth.
Beat on medium speed for about 3 minutes.
Use a large ice cream scoop to fill each cupcake liner about halfway full.
Bake for 15–18 minutes or until a toothpick inserted into the center comes out clean.
Let cupcakes cool completely on a wire rack.
Beat butter until creamy.
Gradually add powdered sugar, cinnamon, and heavy cream.
Beat until fluffy.
Frost the cooled cupcakes generously, then sprinkle with a mix of cinnamon and brown sugar.
