Into a bowl sift together the flour and the salt and blend in the butter until the mixture resembles coarse meal. In another bowl beat together the egg and the sour cream, add to the flour mixture, and stir the mixture with a fork until it forms a dough. Form the dough into a ball, dust it with flour, and flatten it slightly. Chill the dough, wrapped in waxed paper, for at least 1 hour or overnight.
Roll the dough into a rectangle 1/8-inch thick and trim it into a strip whose width is equal to the length of 1 frankfurter. Spread the strip with a thin layer of the mustard and arrange 1 frankfurter lengthwise along a short side. Roll the dough around the frankfurter to just enclose it, leaving a 1/2-inch overlap at the seam, and cut off the dough-enclosed frankfurter from the rest of the rectangle. Seal the seam on the frankfurter, cut the frankfurter crosswise into thirds, and arrange the pieces on an ungreased baking sheet. Roll up the remaining frankfurters in the remaining dough in the same manner, re-rolling the dough scraps and spreading the dough with the mustard when necessary, and cut them into thirds. (The frankfurters may be made 2 days ahead up to this point and frozen, covered tightly. Let the frankfurters thaw before baking them.)
Brush the dough with the egg wash, sprinkle it with the caraway seeds, and bake the frankfurters in a preheated 375 degree oven for 20 minutes, or until the dough is golden and flaky. Serve the frankfurters warm or at room temperature.