In a large mixing bowl, cream together the softened butter and cream cheese until smooth and well-combined. Gradually mix in the sifted flour and kosher salt until a cohesive ball of dough forms.

Transfer the dough to a clean surface and gently shape it into a flattened disc. Wrap the disc in plastic wrap and refrigerate for 30 minutes to firm up the dough, making it easier to roll out.

After chilling, generously flour a flat surface and place the dough on it. Lightly dust the top of the dough with flour and roll it into a rectangle approximately 10 inches by 12 inches. Sprinkle with more flour as needed, then roll the dough further into a larger rectangle, about 10 inches by 30 inches.

Spread the raspberry jam evenly over the surface of the dough, leaving a 1/2-inch border along one of the shorter edges uncoated to help seal the log. Carefully roll the dough into a log, starting at the jam-covered end, and finish with the unjammed edge to seal. Refrigerate the rolled dough log for 15 minutes to firm it up.

Preheat your oven to 375°F (190°C). Remove the chilled log from the refrigerator and slice it into 1/4-inch thick cookies using a sharp knife or a piece of sewing thread.

Arrange the sliced cookies flat on a parchment-lined baking sheet, ensuring they are spaced apart. Bake in the preheated oven for 17-20 minutes, or until the edges are lightly golden.

Transfer the baked cookies to a wire rack to cool completely. Once cooled, dust them generously with powdered sugar before serving.