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1
Preheat oven to 350u00b0. Line bottom and sides of a 13- x 9-inch pan with heavy-duty aluminum foil, allowing 2 to 3 inches to extend over sides; lightly grease foil.
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2
Pulse 2 cups flour, 1/2 cup powdered sugar, and 1 cup butter in a food processor 5 to 6 times or until mixture is crumbly. Press mixture onto bottom of prepared pan.
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3
Bake at 350u00b0 on an oven rack one-third up from bottom of oven 25 minutes or just until golden brown.
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4
Whisk together eggs and next 3 ingredients in a large bowl until blended.
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5
Combine baking powder, salt, and remaining 1/4 cup flour; whisk into egg mixture until blended. Pour lemon mixture into prepared crust.
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6
Pulse 2 cups blackberries and 1/2 cup granulated sugar in a food processor 3 to 4 times or until blended. Transfer mixture to a small saucepan.
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7
Cook over medium-low heat, stirring often, 5 to 6 minutes or until thoroughly heated. Pour through a fine wire-mesh strainer into a bowl, gently pressing blackberry mixture with back of a spoon; discard solids.
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8
Drizzle over lemon mixture in pan.
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9
Bake at 350u00b0 on middle oven rack 30 to 35 minutes or until filling is set. Let cool in pan on a wire rack 30 minutes.
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10
Lift from pan onto wire rack, using foil sides as handles, and let cool 30 minutes or until completely cool. Remove foil, and cut into 24 (2-inch) squares; sprinkle with powdered sugar. Garnish, if desired.
