MAKE THE DOUGH
Heat oven to 400F degrees with the rack placed in the center of the oven.
In a food processor, put the flour, sugar and salt in and pulse several times to combine. Alternatively, use a whisk for the dry ingredients and a pastry cutter to incorporate the butter and ice water. Scatter the cut butter over the dry ingredients and continue pulsing until the mixture is in pea-sized pieces. Add a tablespoon or so of ice water and pulse again, repeating this process until all the water is incorporated, stopping to scrape the sides to make sure the whole mixture is processed. The dough should hold together when you pinch it. Turn the dough out to and gather it into a ball, then flatten into a disc and wrap in two pieces of parchment paper, one for the top of the dough round, one for the bottom. With a rolling pin, flatten the dough again into a 12" disc. Place on a plate and refrigerate for at least 2 hours. Remove dough from the fridge and let it sit for about 10 minutes, until it's slightly soft and easy to work with. Place the 12" dough disc on a place on a rimmed cookie sheet lined with parchment paper. MAKE THE BLACKBERRY FILLING
In a medium bowl, place the berries, lemon zest, sugar, vanilla, flour, sea salt and cinnamon together gently with a spoon until just combined and the sugar and flour are dissolved, being careful not to damage the shape of the fruit.
With your fingers, sprinkle the cookie or graham cracker crumbs on the dough, leaving a 2" border around the edges.
Spoon the berries into the center of the dough, including any juices, leaving a 2" border, piling the berries a bit more in the middle.
Dot the berries with the 1 1/2 tbsp. cold butter.
Roll the dough up onto the berries, crimping the folds together with your fingers to make a galette round.
Brush the dough very lightly with a little cold water, using your fingers or a pastry brush. and then sprinkle with a tablespoon or so of fine sugar.
Bake for 45-55 minutes, until the dough is golden brown and the juices are running and bubbling down the side a little.
Remove from the oven and let it cool to room temperature before serving.Serve with whipped cream or vanilla ice cream, if desired