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

In a small bowl, whisk together the melted butter and brown sugar until well combined. Divide this mixture evenly among the 12 cupcake liners, placing a spoonful in the bottom of each.

Evenly distribute the fresh blackberries over the brown sugar mixture in each liner. Aim for about 3-5 blackberries per cupcake, depending on their size, ensuring they fit relatively flat.

In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

In a large mixing bowl, using an electric mixer, cream together the softened butter and granulated sugar on medium-high speed until light and fluffy, about 2-3 minutes.

Add the eggs one at a time, beating well after each addition. Scrape down the sides of the bowl as needed. Beat in the vanilla extract.

Gradually add the dry ingredient mixture to the wet ingredients, alternating with the milk, beginning and ending with the dry ingredients. Mix on low speed until just combined, being careful not to overmix. A few lumps are okay.

Carefully spoon the cupcake batter over the blackberry topping in each prepared liner, filling each about two-thirds full.

Bake for 18-22 minutes, or until a wooden skewer inserted into the center of a cupcake comes out clean. The cupcakes should be golden brown and springy to the touch.

Remove the muffin tin from the oven and let the cupcakes cool in the tin for 5 minutes. This is crucial for the topping to set slightly.

After 5 minutes, carefully invert the cupcakes onto a wire cooling rack. If any blackberries stick to the liner, gently reattach them to the top of the cupcake. Let them cool completely before serving.