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1
Preheat the oven to 375u00b0F. Line a 8x8 inch baking dish with tin foil, leaving an overhang on all sides and lightly grease the foil. Set aside.
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2
In a food processor or stand mixer, combine the flour, sugar and salt. With the mixer running, add the butter, one piece at a time. Process for 1-2 minutes, or until the mixture resembles damp sand. Remove 1/2 cup plus 2 tablespoons of this mixture and set aside in a medium bowl for the streusel. Press the remaining crumb mixture into the prepared pan, packing it down as tightly and evenly as possible. Bake for 14-18 minutes or until just beginning to brown on the edges.
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3
While the crust is baking, prepare the streusel and the filling. For the streusel, add the brown sugar and rolled oats to the reserved crumbs and stir to combine. Add the butter and use your hands to rub the butter into the crumb mixture, squeezing handfuls of the crumbs together so that it clumps into hazelnut sized lumps. Set aside.
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4
For the filling, toss together the blackberries, sugar, flour and lemon juice in a medium bowl until combined.
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5
Spread the filling on top of the hot crust. Spread the streusel evenly across the filling. Bake the bars for 25-30 minutes, until the streusel is golden brown and the filling is bubbling around the edges. Remove from the oven and allow to cool completely in the pan (1-2 hours) before lifting the bars out by the foil overhang and slicing into bars.
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6
Store in an airtight container at room temperature for up to 3 days. The bars are best the first day, after that the shortbread crust starts to soften.
