To make the crust: Whisk together the flour, salt, and baking powder., 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., Stir in the sour cream. The dough will be craggy, but cohesive., Turn the dough out onto a well-floured surface, and bring it together, if necessary, with a few quick kneads., Pat the dough into a rough square, then roll it into an 8" x 10" rectangle., 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°., 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., 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., 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)., 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., 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., 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., If desired, for a tighter seal, brush two adjoining edges of each square with 1 egg beaten with 1 tablespoon cold water., Place about 2 teaspoons filling slightly off-center in each square; a heaping teaspoon cookie scoop works well here., Fold the pastries in half diagonally, to create triangular turnovers. Press the edges with a fork to seal., 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., Remove the turnovers from the oven, and cool on a rack. Serve warm, or at room temperature.