In a medium bowl, stir together dry topping ingredients until combined, then stir in melted butter until damp crumbs are formed and the mixture sticks together into chunks when you squeeze it between your fingers. Set aside.
Preheat oven to 350F and line a 10" x 8" square pan with parchment paper and set aside.
In a large bowl, cream together sugar and butter until light and fluffy. Beat in vanilla, followed by the eggs, adding them one at a time until mixture is smooth.
In a medium bowl, whisk together flour, baking powder, baking soda and salt. Add to sugar mixture, alternating with buttermilk in two or three additions, until well blended.
Add fresh berries and fold in by hand.
Batter will be quite thick. Spread evenly into prepared pan.
Stir jam in a small bowl until smooth and drop by spoonfuls onto the cake batter. Gently swirl through with a knife. Top with crumb mixture, clumping it by squeezing it between your fingers while you work and spreading it into as even a layer as possible.
Bake for 25-35 minutes or until toothpick inserted into the center comes out clean.
Cool on a wire rack for at least 30 minutes before slicing.