In a medium saucepan over medium heat, combine the chopped peaches, granulated sugar, cinnamon, and lemon juice. Stir gently and simmer until the mixture thickens into a syrup, about 5-7 minutes. In a small bowl, mix the cornstarch with a bit of water, then add it to the peach mixture, cooking for another minute until it thickens further. Remove from heat and refrigerate for at least 2 hours.

Prepare your streusel topping by combining cold butter, brown sugar, all-purpose flour, and cinnamon in a bowl. Using a fork or your fingertips, blend until the mixture resembles coarse crumbs. Spread the streusel on a baking sheet and freeze while you prepare the cupcakes.

Preheat your oven to 350ºF (175ºC) and line a cupcake pan with liners. In a large bowl, whisk together the all-purpose flour, baking powder, baking soda, fine sea salt, cinnamon, and nutmeg. In another bowl, beat the unsalted butter, brown sugar, and vegetable oil until fluffy, around 3-4 minutes. Add the eggs one at a time, mixing in the vanilla extract before gradually incorporating dry ingredients with milk.

Scoop the cupcake batter into the prepared liners, filling them about two-thirds full. This ensures there’s room for the cupcake to rise without spilling over. Bake in the preheated oven for about 18 minutes, or until a toothpick inserted into the center comes out clean and the tops spring back when lightly touched.

Once baked, remove the cupcakes from the oven and allow them to cool in the pan for about 5 minutes. Then transfer them to a wire rack to cool completely.

In a mixing bowl, beat the unsalted butter until it’s light and fluffy, about 3-5 minutes. Gradually add the powdered sugar and spiced cinnamon, mixing until well combined. If the frosting is too thick, adjust the consistency by adding heavy cream a tablespoon at a time until creamy and spreadable.

Once the cupcakes are fully cooled, use a sharp knife to core out a small section from the top of each cupcake. Fill the center with the chilled peach filling, then pipe the buttercream around the outer edge. For a delightful finishing touch, add a dollop of peach filling in the center and sprinkle with cold streusel on top.