Melt butter in a saucepan over medium heat, then add diced apples, brown sugar, cinnamon, and lemon juice. Cook for 5 minutes, stirring occasionally, until the apples soften. Stir in the cornstarch slurry and cook for another 1–2 minutes until the mixture thickens. Remove from heat and set aside to cool.

Preheat the oven to 350°F (175°C) and line a muffin tin with cupcake liners. In a bowl, whisk together the dry ingredients. In another large bowl, beat butter, granulated sugar, and brown sugar until light and fluffy. Add eggs one at a time, then mix in vanilla extract. Alternately add the dry ingredients and milk, mixing until just combined.

Fill the cupcake liners about ⅔ full with batter. Bake for 18–20 minutes or until a toothpick inserted into the center comes out clean. Let the cupcakes cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely.

Beat butter in a large bowl until smooth. Gradually add powdered sugar and mix until fluffy. Stir in vanilla extract, cinnamon, and milk, beating until the frosting reaches a creamy consistency.

Use a small knife or cupcake corer to remove the center of each cupcake. Fill the hole with the cooled apple filling. Pipe or spread the buttercream frosting on top.