Preheat oven to 350°F (175°C). Grease and flour three 8-inch round cake pans.

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

In a large bowl, beat butter and sugar until light and fluffy. Add eggs one at a time, beating after each addition. Mix in vanilla.

Alternate adding dry ingredients and buttermilk to the butter mixture, beginning and ending with dry ingredients.

Fold in blackberry jam and gently fold in floured blackberries.

Divide batter evenly among prepared pans and bake 25–30 minutes, or until a toothpick inserted in the center comes out clean.

Cool cakes in pans for 10 minutes, then transfer to wire racks to cool completely.

For frosting: Beat cream cheese and butter until smooth. Gradually add powdered sugar, then mix in blackberry jam and vanilla until fluffy.

Frost cooled cake layers, stacking them evenly. Spread frosting over the top and sides.

Garnish with fresh blackberries and mint leaves before serving.