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To make the crust: Whisk together the flour, salt, and baking powder.
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Cut the cold butter into pats, and work it into the flour mixture till it's unevenly crumbly, with larger bits of butter remaining intact.
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Stir in the sour cream. The dough will be craggy, but cohesive.
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Turn the dough out onto a well-floured surface, and bring it together, if necessary, with a few quick kneads.
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Pat the dough into a rough square, then roll it into an 8" x 10" rectangle.
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Dust both sides of the dough with flour. Starting with one of the shorter (8") ends, fold it in thirds like a business letter, flip it over (so the open flap is on the bottom), and turn it 90°.
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Roll the dough into an 8" x 10" rectangle again. Fold it in thirds, wrap in plastic, and place in the refrigerator to chill for at least 30 minutes (or overnight) before using.
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While the dough is chilling, make the filling. Mix the sugar and ClearJel until well combined. If you're using cornstarch, mix it with enough cold water to dissolve.
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Add the sugar mixture to the raspberries, tossing to combine. Stir in the vanilla and cinnamon (and the cornstarch/water mixture, if you're using cornstarch).
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Heat the mixture in a saucepan over very low heat, stirring, till the berries soften and fall apart. The mixture will be thick and jam-like, even though it doesn't really warm up much; this will take under 5 minutes. If you use cornstarch, cook and stir till the mixture bubbles and thickens. You can prepare the filling up to several days before; cover and refrigerate till you're ready to use it.
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You can also do this in a microwave; heat till the berries soften, then stir till they fall apart and the mixture thickens, like jam.
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To assemble the turnovers: Once the filling has cooled to room temperature (or has been prepared ahead of time and refrigerated), preheat the oven to 400°F. Roll the chilled dough into a 16" square. Cut sixteen 4" squares.
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If desired, for a tighter seal, brush two adjoining edges of each square with 1 egg beaten with 1 tablespoon cold water.
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Place about 2 teaspoons filling slightly off-center in each square; a heaping teaspoon cookie scoop works well here.
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Fold the pastries in half diagonally, to create triangular turnovers. Press the edges with a fork to seal.
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Place the turnovers on a baking sheet, preferably one lined with parchment to catch any spills. Bake in the preheated 400°F oven for 20 to 25 minutes, or until they're a deep, golden brown; you may see some of the filling beginning to ooze out.
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Remove the turnovers from the oven, and cool on a rack. Serve warm, or at room temperature.