Preheat oven to 325F (160C). Line a baking sheet with parchment. Place butter and sugar in a large bowl. Using a wooden spoon, beat together until creamy. In a bowl, stir flour with salt. Gradually stir into butter mixture just until combined. Divide into 4 portions. Roll into 4 thin logs about 8 in. (20 cm) long. Refrigerate at least 15 min. Place cold logs on a large parchment-lined baking sheet, about 2 in. (5 cm) apart. Using the edge of a rubber spatula, make a thin groove down the centre of each log.
Bake in centre of preheated oven until lightly golden around the edges, 15 to 18 min. Remove from oven and, slicing on the diagonal, cut crosswise into 1-in. (2.5-cm) pieces. Leave on sheet for 5 min. Then remove to a wire rack to cool completely. Before serving, fill each with 1/4 tsp (1 mL) jam.