Preheat Oven and Line Tin:

Preheat oven to 350°F (175°C). Line a 12-cup muffin tin with parchment or paper cupcake liners.

Whisk Dry Ingredients:

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

Cream Butter and Sugar:

In a large bowl, beat softened butter and granulated sugar until pale and fluffy, about 3-4 minutes on medium speed.

Add Eggs and Vanilla:

Add eggs one at a time, mixing well after each. Stir in vanilla extract.

Alternate Dry and Wet:

Gradually add flour mixture to creamed butter mixture, alternating with milk, beginning and ending with dry ingredients. Stir just until combined.

Divide Batter and Swirl:

Divide batter into two bowls. Fold blackberry puree gently into one. Using alternating spoonfuls of vanilla and blackberry batters, fill cupcake liners about 2/3 full. Lightly swirl with a toothpick.

Bake and Cool Cupcakes:

Bake for 18-20 minutes or until a toothpick inserted in center comes out clean. Rotate pan halfway through. Cool in pan 5 minutes, then transfer to wire rack.

Make and Divide Frosting:

Beat softened butter until creamy. Gradually add powdered sugar, mixing with heavy cream as needed until smooth and fluffy. Divide frosting in two bowls. Stir vanilla into one, blackberry puree into the other. Spoon both frostings side-by-side into a piping bag fitted with a star tip.

Pipe Frosting on Cupcakes:

Once cupcakes are completely cool, pipe frosting onto each in a gentle spiral from center outward.

Garnish with Blackberries:

Top each frosted cupcake with a fresh blackberry and a mint leaf if desired.