Preheat oven to 350°F (180°C). Line a cupcake pan with paper liners.

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

In another bowl, beat the melted butter and sugar with a mixer until well combined. Add the eggs, one at a time, beating well after each addition. Mix in vanilla, lemon juice, and lemon zest.

Alternate adding the flour mixture and milk, mixing just until combined. Do not overmix.

Fill each cupcake liner about two-thirds full.

Bake for about 15 minutes, or until a toothpick inserted in the center comes out clean.

Let cupcakes cool completely.

If your jam has been refrigerated and is too thick, stir well before using so it’s spreadable.

With a small knife, cut a cone-shaped piece out of the center of each cupcake. Fill the cavity with about 1 teaspoon of blackberry jam.

Beat the butter until creamy, about 2–3 minutes.

Add salt and gradually beat in powdered sugar (about 1 cup at a time) until incorporated.

Mix in lemon juice and beat again.

If you prefer a softer consistency, add 1 tablespoon heavy cream at a time until desired texture is reached.

Transfer frosting to a piping bag fitted with a decorating tip. Pipe onto cooled cupcakes.

Garnish with fresh blackberries or lemon zest.