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Preheat oven to 350F and line a 10" x 8" square pan with parchment paper and set aside.
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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.
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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.
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Add fresh berries and fold in by hand.
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Batter will be quite thick. Spread evenly into prepared pan.
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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.
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Bake for 25-35 minutes or until toothpick inserted into the center comes out clean.
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Cool on a wire rack for at least 30 minutes before slicing.
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Serves 12.
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Allow cake to completely cool before icing.