In a small bowl, whisk together the 1/2 cup all-purpose flour, 1/4 cup packed light brown sugar, and 1/2 tsp ground cinnamon. Add the 4 tbsp cold, cubed unsalted butter and cut it in with a pastry blender or your fingertips until the mixture is crumbly and resembles coarse meal. Set aside.

In a small saucepan, combine the 1 cup finely diced peaches, 2 tbsp granulated sugar, 1 tbsp cornstarch, 1 tsp lemon juice, and 1/4 tsp ground cinnamon. Cook over medium heat, stirring constantly, until the mixture thickens and comes to a gentle boil (about 3-5 minutes). Remove from heat and let cool completely.

Preheat your oven to 375°F (190°C). Line a 12-cup muffin pan with paper cupcake liners.

In a medium bowl, whisk together the 1 1/2 cups all-purpose flour, 1 tsp baking powder, 1/2 tsp baking soda, 1/4 tsp salt, and 1/2 tsp ground cinnamon. Set aside.

In a large bowl, using an electric mixer, cream together the 1/2 cup softened unsalted butter and 1 cup granulated sugar until light and fluffy (2-3 minutes). Beat in the 2 large eggs one at a time, mixing well after each addition. Stir in the 1 tsp vanilla extract.

Gradually add the dry ingredient mixture to the wet ingredients, alternating with the 1/2 cup buttermilk. Begin and end with the dry ingredients. Mix until just combined; be careful not to overmix.

Divide about half of the cupcake batter evenly among the 12 prepared muffin cups (about 1.5-2 tablespoons per cup).

Spoon about 1 tablespoon of the cooled peach filling into the center of each cupcake, gently pressing it down slightly.

Top with the remaining cupcake batter, dividing it evenly among the cups to cover the peach filling.

Generously sprinkle the prepared cinnamon streusel topping over the top of each cupcake.

Bake for 20-25 minutes, or until a wooden skewer inserted into the center of a cupcake (avoiding the peach filling) comes out clean and the streusel is golden brown.

Let the cupcakes cool in the muffin pan for 5 minutes before transferring them to a wire rack to cool completely.