FOR THE FILLING: Place the sugar and cornstarch in a small saucepan and mix together. Add the water and 1 C. blackberries, mixing into the sugar mixture and smashing up the berries a bit.
Heat the pan on medium heat, cooking and stirring until the blackberries start to give up their juice, pressing them down to help pop them as you stir. Make sure you stir frequently so that the sugar doesn't stick to the bottom. The sauce will become thick and glossy, with most of the berries burst and deflated.
Remove from heat and stir in the remaining 1½ C. blackberries. When the filling is cool, stir in the lemon juice.
Refrigerate ir freeze the filling. It should be cold before using it to fill the crusts.
FOR THE CRUST: Place the flour, sugar, salt, and baking powder in the bowl of a food processor. Place the lid on and pulse the mixture a few times to blend.
Add the butter and shortening to the crust, 1 T. at a time, pulsing a few times with each addition(don't run the processor consistently). When the butter and shortening are added, pulse a few times more, or until the butter is cut down to pea sized pieces.
Mix together the cold water, vinegar, egg yolk, and ice cubes. Let chill for a minute or two. Add 3½-4 T. of the mixture to the dough 1 T. at a time, pulsing a few times after each addition until the dough comes together, but is not too wet. Chill immediately while you prepare to roll out the dough.
Place a sheet of waxed paper on your work surface. Take half of the dough out of the fridge, place on the paper and flatten into a disc.
Place another piece of waxed paper on top of the dough. Roll the dough out thin, at least ⅛" thick, or thinner, if desired.
Remove the top sheet of waxed paper and cut circles out of the dough that are at least 2-2½" in diameter. Place dough circles on a baking sheet lined with a silpat mat or parchment paper.
Roll out the other half of the dough in the same manner. Cut out circles of dough that are at least ½" larger than the dough circles on the baking sheet.
Working one at a time, place 1 teaspoon of mascarpone in the center of a dough circle, pressing each dollop down a bit with a spoon. Top with 2 T. of the cooled filling. Gently place one of the larger dough circles on top of the filling and seal the edges by pressing your fingertips around the edges of the dough circle. Press the tines of a fork around the edge of the circle to fully seal in the filling.
Repeat this procedure with the remaining dough and filling.
Place the egg and milk in a small bowl and beat well.
Using a pastry brush, brush the tops of the pies with the mixture.
Bake the pies at 350 degrees for 10-12 minutes, or until the crust is golden brown.
Remove from oven and cool on wire rack.
Recipe by The Kitchen McCabe at https://www.thekitchenmccabe.com/2014/07/07/blackberry-mascarpone-hand-pies/