Bake the cookies: In a medium bowl, combine all-purpose flour, salt, and baking powder. Whisk quickly to combine and set aside. In a bowl of an electric mixer fitted with a paddle attachment, over medium/high speed, beat the butter, cream cheese, and granulated sugar until light and fluffy. This should take about 2-3 minutes. Add the egg, followed by the vanilla extract and beat until combined. Stop mixer and scrape the bowl. Turn the mixer speed to low and add the flour all at once. Beat until just combined. Remove bowl from the mixer. Divide the dough into four parts and wrap in plastic wrap. Chill the dough in the freezer for at least an hour. Preheat the oven to 350° Fahrenheit. Line two cookie sheets with parchment paper. Generously sprinkle your counter or workspace with all-purpose flour. Remove one piece of dough from the fridge. Place over your floured surface and roll the dough to about 1/8 inch thickness. Use a bench cutter or a large spatula periodically to ensure the dough does not stick to the surface. Cut dough with a lightly floured round cutter (I used a fluted round cutter). With a heart-shaped cookie cutter, cut out hearts in the center of half the cookies. Place about 1 inch apart on the parchment-lined cookie sheets. Gather the scraps, wrap in plastic wrap and place in the refrigerator to chill. Repeat with remaining dough. Place in the oven and bake for 7-8 minutes until the cookies are a very light golden brown and set. Remove cookies from oven and allow to cool on the cookie sheets for about 5 minutes, then transfer to a wired rack to cool completely. Fill the cookies: Lightly brushed each heart-shaped cookie with a tiny little bit of water. Sprinkle confectioners’ sugar over the heart-shaped cookies. Flip the uncut cookies so the bottom is face up. Spread ½ - 1 teaspoon of strawberry jam on each cookie. Top with the heart-shaped cookie. Makes about 42 sandwich cookies.