In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

In a large bowl, using an electric mixer, cream together the softened butter and granulated sugar until light and fluffy (2-3 minutes).

Beat in the egg and vanilla extract until well combined.

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix. The dough will be thick.

Divide the dough in half, flatten each half into a disk, wrap tightly in plastic wrap, and refrigerate for at least 30 minutes (or up to 2 days). This step is crucial for easy handling and preventing spread.

Preheat oven to 375°F (190°C). Line two baking sheets with parchment paper.

On a lightly floured surface, roll out one disk of dough to about 1/4 inch thickness. Use heart-shaped cookie cutters to cut out shapes.

Carefully transfer cookies to the prepared baking sheets, leaving about 1 inch between them. Gather dough scraps, re-roll, and cut more cookies until all dough is used.

Bake for 10-12 minutes, or until the edges are lightly golden. Repeat with the second disk of dough.

Let cookies cool on the baking sheets for 5 minutes before transferring to a wire rack to cool completely.

While cookies are cooling, prepare the glaze. In a medium bowl, whisk together the sifted powdered sugar, milk, and vanilla extract until smooth. The glaze should be thick but pourable. If too thick, add more milk 1 teaspoon at a time; if too thin, add more powdered sugar 1 tablespoon at a time.

In a small microwave-safe bowl, warm the raspberry jam for 15-20 seconds until slightly runny.

Add the warmed raspberry jam to about 1/4 cup of the prepared white glaze. Stir gently to create a raspberry-colored glaze with visible swirls. (Add red food coloring if a deeper pink is desired).

Once cookies are completely cool, dip the top of each cookie into the plain white glaze, letting excess drip off. Place back on the wire rack.

Immediately, before the white glaze sets, use a spoon or a piping bag to drizzle small amounts of the raspberry swirl glaze onto the white glaze. Use a toothpick to gently swirl the raspberry into the white glaze to create a marble effect.

Sprinkle with Valentine's sprinkles while the glaze is still wet.

Let the glazed cookies set at room temperature for at least 1-2 hours, or until the glaze is firm.