Whisk together flour, zest, sugar and salt in the bottom of a large bowl.

Using a pastry blender, cut the butter into the flour until the biggest pieces of butter are the size of tiny peas.

Gently stir in 3/4 cup buttermilk with a rubber spatula, mixing it until a bit of a mass forms, then knead it two or three times to form a ball.

If it doesn’t come together, add remaining 1/4 cup buttermilk, 1 tablespoon at a time, until it does, then gently knead again. (I had to add all of the remaining 1/4 cup)

Divide dough in half. Wrap each half in plastic wrap and flatten into a disc. Chill in fridge for at least an hour or up to two days.

Taste a few blueberries: If they are sweet, use closer to 1/2 cup sugar. If they are not, you may need up to 3/4 cup of sugar.

Combine the blueberries with the sugar in a medium saucepan. Simmer on low heat until sugar has melted and berries begin to burst. Stir occasionally. This will take about 5 - 7 minutes.

Combine cornstarch and water in a (very) small bowl. Once blueberries have begun to burst and sugar is melted, smash any remaining berries to a pulp and add the cornstarch mixture to the pot. Cook over medium heat, stirring frequently, until it has thickened.

Add the lemon zest, butter and salt, cook for another 30 - 45 seconds. Remove from heat and cool completely.

Preheat oven to 400°F and line 3 baking sheets with parchment paper.

Generously flour the counter top, then unwrap one of your disks of dough. Gently begin rolling it out, starting from the center and pushing outward. The dough will be pretty firm when you begin, so be patient. (it will warm quickly, so not too patient!)

Keep rolling from the center out, turning the dough as you go, until it is about 1/8 inch thick. If the butter in the dough begins to get too warm and it starts to stick to the pin or gets goopy, slide it onto a baking sheet and put it in the freezer for a few minutes to firm up again.

Once dough is rolled out, cut out with a 2 1/2 - 3 inch cutter (whatever shape you have) or use a knife and cut into squares.

Prepare egg wash, get out the pastry brush & set aside.

Lightly brush half of your circles or squares with the egg wash, taking care to get the edges, cut vents in the center of the other half of your circles/squares.

Place 1 rounded teaspoon of filling in the center of the egg washed halves and top with the vented pieces. (don't overfill!) Seal with your fingertips or press with a fork along the edges.

Brush with the egg wash on top and sprinkle with coarse sugar. Chill in refrigerator before baking if dough begins to get mushy. Repeat with remaining disk of dough, filling, etc.

Place on baking sheet 1 - 2 inches apart and bake for 15 - 18 minutes until golden and puffed.

Cool on wire rack.