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1
Preheat oven to 350 degrees F (175 degrees C). Line an 8-inch baking dish with parchment paper, leaving an overhang on two sides.
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2
Combine 1/2 cup white whole wheat flour, oats, 6 tablespoons butter, brown sugar, 1/2 teaspoon vanilla extract, and 1/4 teaspoon salt in a small bowl; mix with a fork to make a crumbly topping. Place in the refrigerator.
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3
Combine 1/2 cup butter and confectioners' sugar in a bowl; beat with an electric mixer until light and creamy. Add eggs and egg yolk one at a time, mixing well after each addition. Beat in lemon zest, rosemary, and 1/2 teaspoon vanilla extract.
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4
Mix 3/4 cup white whole wheat flour, baking powder, and 1/8 teaspoon salt together in a separate bowl. Add to the creamed butter mixture in 2 to 3 batches, mixing well after each addition, until a thick dough forms.
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5
Spread dough in the bottom of the prepared pan. Arrange blackberries evenly on top. Stir crumb topping and sprinkle evenly over blackberries. Press down gently to pack topping down.
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6
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 40 to 45 minutes. Transfer to a wire rack; let cool for 1 hour.
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7
Lift bars out of the pan using the parchment paper edges. Cut into 12 pieces using a long serrated knife.
