In a medium bowl, combine flour, salt, sugar, and baking powder. Set aside.
In the bowl of a food processor, combine flour mixture, butter, and cream cheese. Add 1 teaspoon ice cold water and pulse until the mixture just comes together and resembles small peas, adding more water 1/2 teaspoon at a time, if needed.
Turn dough out onto counter and lightly press together with your hands to form a disc. Wrap tightly with plastic wrap and chill for at least 1 hour.
When ready to bake, preheat oven to 375 F. Remove dough from refrigerator and place on a surface lightly dusted with flour. Roll to 1/8-inch thick, then cut six rectangles about 4 1/2 inches long and 2 1/2 inches wide. Reroll scrap dough to get two more rectangles, for a total of 8.
In the center of 4 of the dough rectangles, place 1/4 cup jam, leaving 1/2-inch border around the edges. Using your finger, use a small amount of water to wet the edge, then take one of the remaining rectangles of dough and place on top of the jam. Press down along the edges to form a seal. Repeat with remaining dough to make 4 tarts.
Use a paring knife to make a small cross in the center, piercing the top layer of dough. Transfer tarts to a baking sheet and bake until golden brown, about 15 minutes. Remove from oven and let cool completely.
Meanwhile, in a small bowl, whisk together powdered sugar and milk until well combined. Spoon about a tablespoon of frosting onto each baked and cooled tart and lightly spread. Top with sprinkles, if desired.