Prepare the Pecan Pie Filling: In a medium saucepan, whisk together cornstarch, sugar, and salt. Gradually pour in the milk while stirring consistently to prevent lumps. Cook over medium heat for 5-7 minutes until thickened. Remove from heat and stir in pecans and butter. Let cool.

Make the Cupcake Batter: In a mixing bowl, whisk together flour, baking powder, baking soda, and cinnamon. In another bowl, cream butter with brown and granulated sugars until light and fluffy. Add eggs, vanilla extract, and sour cream, mixing until combined. Fold in dry ingredients.

Fill and Bake: Preheat oven to 350°F (175°C). Line cupcake tin with paper liners. Fill each liner two-thirds full with batter and bake for 15-18 minutes. Let cool in the pan for 10 minutes before transferring to a wire rack.

Prepare the Brown Sugar Buttercream: Cream together softened butter and brown sugar until light and fluffy. Gradually add powdered sugar and milk until frosting is smooth and spreadable.

Assemble the Cupcakes: Once cooled, create a well in the center of each cupcake and fill with pecan pie filling. Pipe or spread buttercream on top.