To make the dough: In a medium mixing bowl, whisk together the flour, salt, and baking powder. Work in the butter until the mixture resembles coarse cornmeal, then stir in the sour cream., Turn the mixture out onto a lightly greased work surface and knead it briefly, just until it holds together. Roll it into a 12" x 16" rectangle, fold in three (like a business letter), flip the dough over and give it a 90°; turn, and repeat the rolling and folding process., Wrap tightly and chill for several hours, or overnight., Preheat the oven to 400°F. Lightly grease a baking pan or line it with parchment., Remove the dough from the refrigerator and unwrap it., In a small bowl, mix together the sugar and cinnamon., Sprinkle both sides of the dough with the cinnamon sugar, and roll it into a 12" x 16" rectangle, sprinkling with additional sugar as needed to keep the dough from sticking. Once the dough is at the proper size, sprinkle any remaining sugar over it and roll over it with your pin to press it into the dough., Starting with a long edge, roll the dough into a log; finish with the seam underneath. Slice the dough into 48 pieces, each 1/3" wide. Place the "snails" on the prepared baking sheet., Bake the snails for 8 minutes, until the sugar has melted on the bottom. Then turn over and bake for 5 minutes longer, for a very crisp pastry. For a softer version, bake for 10 to 12 minutes without turning., Cool on the baking sheet for 3 minutes, then transfer to a rack to cool completely. Store for up 2 weeks in an airtight container.