Whisk together the cornstarch, sugar, and salt in a saucepan over medium heat. Gradually stir in milk and cook, stirring constantly, until the mixture thickens and bubbles, about 5-7 minutes. Remove from heat and mix in chopped pecans and butter until incorporated. Allow to cool.

In a large bowl, whisk together flour, baking powder, baking soda, and cinnamon. In a separate bowl, cream butter with brown and granulated sugars until light and fluffy. Add eggs, vanilla, and sour cream, blending until smooth. Gradually add dry mixture to wet ingredients until just combined.

Preheat oven to 350°F (175°C) and line a cupcake tin with liners. Fill each liner with batter about two-thirds full. Bake for 15-18 minutes, or until a toothpick inserted comes out clean. Cool in the tin for about 10 minutes, then transfer to a wire rack.

In a mixing bowl, cream together softened butter and brown sugar until fluffy. Gradually add powdered sugar and a splash of milk, beating until creamy. Adjust with more milk as needed for desired consistency.

Once the cupcakes have cooled, use a small knife or cupcake corer to remove a small portion from the center. Fill each well with pecan pie filling, then pipe or spread brown sugar buttercream on top.