Preheat your oven to 350°F (175°C) and line a muffin tin with cupcake liners.
In a bowl, whisk together flour, sugar, baking powder, baking soda, and salt.
In another bowl, beat the butter until creamy. Add eggs one at a time, then mix in vanilla extract.
Gradually add the dry ingredients to the wet ingredients, alternating with milk. Mix until just combined.
For the peach filling, combine diced peaches, sugar, lemon juice, and cornstarch in a saucepan. Cook over medium heat until thickened.
Fill each cupcake liner halfway with batter, add a spoonful of peach filling, then top with more batter.
Bake for 20-25 minutes or until a toothpick inserted into the center comes out clean. Let cool before serving.