1. Whisk together flour, baking powder, and salt. Beat together butter and sugar with a mixer on medium speed until pale and fluffy, about 3 minutes. Beat in egg and vanilla. Reduce speed to low, and gradually add flour mixture, beating until just incorporated. Remove from the mixer and and knead until a dough forms.

2. Divide dough into 3 or 4 pieces, depending on how many shades of pink you would like the cookies to be. Working with 1 piece of dough at a time, add food coloring, 1 drop at a time, mixing and kneading to combine, until desired shade is reached; the more food coloring you add, the darker the color will be. Flatten each piece of colored dough into a disk, and wrap each in plastic wrap. Refrigerate until firm, at least 1 hour and up to overnight. Bring to room temperature, about 10 minutes, before rolling.

3. Preheat oven to 325 degrees with racks in top and lower thirds. Roll out each disk of dough between 2 sheets of lightly floured parchment to just under 1/4 inch thick, adding more flour as needed to keep dough from sticking. Cut out 1-, 1 1/2-, and 2-inch heart shapes, making sure you have even numbers in each color and size for a total of thirty-six 1-inch, thirty-six 1 1/2-inch, and thirty 2-inch cookies, rerolling scraps once. Place cookies 1 inch apart on parchment-lined baking sheets, and freeze until very firm, about 15 minutes.

4. Bake until barely golden brown around edges, about 8 minutes for 1-inch cookies, 10 minutes for 1 1/2-inch cookies, and 12 minutes for 2-inch cookies, rotating halfway through. Let cookies cool completely on baking sheets set on wire racks. Spread or pipe (using a pastry bag and a small plain tip) filling onto bottom side of half the cookies, and sandwich with remaining cookies, pressing gently. Cookies can be stored in an airtight container at room temperature up to 1 week.