Preheat the oven to 350°F (175°C). Line a cupcake pan with liners.
In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
In a large mixing bowl, cream butter and brown sugar until light and fluffy (about 4–5 minutes).
Beat in the eggs, one at a time, mixing well after each.
Stir in the vanilla.
Alternate adding the flour mixture and bourbon to the batter, beginning and ending with flour. Mix just until combined.
Gently fold in the blackberries using a spatula.
Divide the batter evenly among cupcake liners, filling about 2/3 full.
Bake for 18–22 minutes or until a toothpick inserted in the center comes out clean.
Cool cupcakes in the pan for 5 minutes, then transfer to a wire rack to cool completely.
For the frosting, beat butter until smooth. Add powdered sugar gradually. Mix in bourbon and salt. Adjust consistency if needed.
Frost the cooled cupcakes and garnish as desired.