To Make the dough: Using an electric mixer at medium-high speed, cream the butter, gradually adding the sugar. Beat in the egg until evenly mixed, then blend in the vanilla extract and salt.
With a wooden spoon, stir the flour into the creamed ingredients, about one third at a time, until evenly blended. The dough may seem soft, but it will firm up when refrigerated.
Divide the dough in half. Flatten each portion into a disk and seal in plastic wrap. Refrigerate overnight.
To create the cookies: Heat the oven to 375 degrees. Cover a sturdy baking sheet with aluminum foil and lightly coat the foil with cooking spray. Between 2 sheets of waxed paper lightly dusted with flour, roll the dough to a 1/4-inch thickness. Remove the top sheet.
Cut out the cookies with a large cookie cutter. Use a spatula to transfer the shapes to the baking sheet, leaving about an inch between cookies. Remove the centers of the cookies with a smaller cookie cutter. (Save the centers to bake later.)
Place a hard candy (I used Jolly Ranchers) in the center of each heart and bake until the cookies start to brown lightly around the edges and the candy is melted, about 8 to 10 minutes. Let the cookies cool on the baking sheet for 5 minutes, then transfer them to wire racks to cool completely. To prevent sticking, line your serving plate or tin with waxed paper and place additional waxed paper between layers. Makes 30.