Prepare the Pecan Pie Filling: In a medium saucepan, whisk together cornstarch, granulated sugar, and salt. Gradually stir in milk, then heat over medium, cooking for 5-7 minutes until the mixture thickens and bubbles. Once thickened, remove from heat and mix in chopped pecans and butter until melted. Let the filling cool while you prepare the cupcake batter.

Make the Cupcake Batter: In a large mixing bowl, combine flour, baking powder, baking soda, and ground cinnamon. In another bowl, cream together softened butter, brown sugar, and granulated sugar until fluffy and light. Add eggs one at a time, mixing well after each addition, then blend in vanilla extract and sour cream. Gradually mix the dry ingredients into the wet until just combined.

Bake the Cupcakes: Preheat your oven to 350°F (175°C). Line a cupcake tin with paper liners and fill each liner about two-thirds full with batter. Bake for 15-18 minutes, or until a toothpick inserted into the center comes out clean. Let the cupcakes cool in the tin for 10 minutes before transferring them to a wire rack to cool completely.

Prepare the Brown Sugar Buttercream Frosting: In a medium bowl, cream the softened butter and brown sugar together until light and airy. Gradually add powdered sugar, mixing until well combined and creamy. Adjust consistency by adding milk, a teaspoon at a time, until it reaches desired texture.

Assemble the Cupcakes: Once the cupcakes are completely cooled, use a small knife or a cupcake corer to create a well in the center of each cupcake. Spoon the cooled pecan pie filling into each well generously. Finally, spread or pipe the brown sugar buttercream on top of each cupcake, creating a delightful swirl.