3 1/2ouncesunsalted butter,very cold and cut into small pieces
2-3tablespoonsice cold water
For The Blackberry Filling
1/2teaspoonfresh-squeezed lemon juice
1teaspoongrated lemon zest
1 1/2teaspoonsturbinado sugar
Optional for serving: vanilla ice cream
Prepare the dough: Add the flour and salt to a food processor, pulsing several times to combine. Add the butter and pulse on and off until the mixture is crumbly. Slowly add the water with the machine running until the dough begins to form a ball (see notes). Wrap in plastic wrap and press flat into a disc. Chill for at least 30 minutes before using so the butter can firm back up.
Place an oven shelf on the center rack, then preheat the oven to 400 degrees Line a sheet pan with parchment paper.
In a large bowl, combine the blackberries, sugar, cornstarch, lemon juice, lemon zest, and salt. You don’t want the filling to sit for very long before going into the oven, so do this right before rolling out the dough.
Roll the dough onto a lightly floured surface until it's approximately 12 inches in diameter. You can optionally roll it a bit thinner and then use a pizza cutter to clean up the edges. Transfer the rolled dough to a baking sheet covered in parchment paper. (The best way to do this is to roll the dough around the rolling pin, then unroll it on top of the baking sheet).
Spread the berries evenly onto the dough, leaving about 2 inches around the edges. Fold the excess dough on top of the cherries in a circle, until you have a tightly formed tart.
Chill the tart for 10 minutes to firm up the dough. Brush the top of the dough with a light coating of egg wash and a sprinkle of turbinado sugar.
Bake for 10 minutes, then lower the oven temperature to 375 degrees. Continue baking for an additional 20 minutes, or until the dough is firm and the cherries are bubbling.
Allow to cool briefly before serving. Top slices with vanilla ice cream, if desired.
When using a food processor to make dough, you want the machine running for the least amount of time possible once you add the water. This will help keep the dough flaky. You can let the dough form a ball, but it will be a tougher crust. If you stop the machine once the ingredients are just combined and flaky, you can press them together by hand for more tender results. I press them together while wrapping the dough in plastic wrap.