Put the flour into a large bowl and whisk in the salt.
Cut the butter into tablespoon-size pieces and add to the flour.
Use a pastry blender to cut the butter into the flour until the mixture resembles medium-coarse flakes. (Feel free to use your hands, rapidly pinching the fat and flour together).If you prefer, you can cut the butter into the flour with a food processor, pulsing rapidly until the butter is in small bits.
Stir in the sour cream with a fork to make a dough that just holds together.
Flour lightly if dough is sticky. Shape into a 1-inch-thick square, wrap tightly in plastic wrap and refrigerate 2 hours or overnight. Or freeze for up to 2 months.
Spread a section of your work surface with granulated sugar or cinnamon sugar.
For cinnamon sugar, stir together 1 cup sugar with 1 tablespoon ground cinnamon.
Put the dough on the sugar, turning it to coat all sides.
Keeping the dough well-sugared, roll it into a 14-inch square.
Sprinkle more sugar on top of the dough.
Determine the center of the dough and mark it lightly with a ruler or the back of a knife.
Fold each side into thirds toward the center, folding so that, rather than meeting in the center, there is a ½-inch space between the edges. Here’s the left side folded. Note the line indicating the center of the dough.
And here’s the right side folded.
Then close the dough along the central space like a book, making a compact 6-layered roll about 2 inches wide and 14 inches long.
Wrap airtight and refrigerate for 1 hour or more, until dough is firm enough to be sliced evenly. Trim away uneven edges.
Adjust an oven rack to the center position and preheat the oven to 400 degrees. Line two large cookie sheets (14 x 17 inches) with aluminum foil, cooking parchment, or silicone baking pan liner. If using USA Pan cookie sheets, lining the pans is unnecessary.
With a very sharp knife, cut the dough into eighteen 3/4-inch-thick slices.
Dip both sides of the slices into sugar and place the palmiers 2 to 3 inches apart on the sheets, 9 palmiers to a sheet.
Refrigerate one sheet and place the second in the oven.
Bake at 375° for 12 to 15 minutes, or until the sugar on the bottom has melted and is well-caramelized.
Turn palmiers over with a wide metal spatula and continue baking until palmiers are evenly and well-browned throughout, 3 to 5 minutes more. Do not underbake. Pastries must be a deep brown color.
Slide the pan liner off the cookie sheet onto your work surface and let palmiers cool a few minutes before transferring them to cooling racks to finish cooling.
Store airtight and serve when very fresh.