Cookies:

Line a half-sheet rimmed baking tray with parchment or a silicone mat. Set aside.

In a large mixing bowl, whisk the flour, baking powder, and fine sea salt.

In a separate bowl or the bowl of a stand mixer, add butter and granulated sugar and mix on high speed until light and fluffy.

Beat in the egg, followed by the vanilla bean paste.

Add the dry ingredients and mix until just combined.

With a wooden spoon, fold in the rainbow sprinkles.

Flatten dough into two discs. Wrap in clingfilm/plastic wrap and transfer to the refrigerator to chill, at least one hour.

On a floured rolling mat, roll the chilled dough to 1/8 – 1/4-inch thickness.

Use a 2 1/2-inch heart-shaped cutter to cut the dough into hearts.

Use a 1-inch heart cutter to cut a smaller heart in the middle of half of the cookies.

Re-roll scraps and repeat with remaining dough.

Transfer to on prepared tray and freeze for 15 minutes. (Alternately, you can stack the cookies between squares of parchment and freeze the stacks, if your trays don’t fit in the freezer.)

While cookies freeze, preheat oven to 350° F / 176° C.

Bake for 12-15 minutes, or until lightly golden brown. Allow cookies to cool on the tray for 10 minutes, then transfer to a wire rack to cool completely.

Repeat with remaining cookies.

While cookies cool, prepare the buttercream. (If you want to work ahead, you can also make the buttercream while the cookie dough chills—it will be okay at room temperature for a few hours.)

 Buttercream:

In a large mixing bowl or the bowl of a stand mixer, whip butter until light and fluffy.

Add the sugar in small increments, alternating with the milk and mixing well after each addition.

Add the vanilla extract and mix until smooth and creamy, about 3-5 minutes. If necessary, add more confectioner’s sugar or milk to adjust the consistency. The buttercream should be thick and creamy, but soft enough to pipe.

Portion buttercream into six individual bowls.

Add 1-2 drops of gel food coloring to each bowl. Stir to create six differently-colored buttercreams. You should have red, orange, yellow, green, blue, and purple.

Transfer each color to its own piping bag.

On a piece of clingfilm/plastic wrap, pipe a stripe of each color, mimicking the color pattern of the rainbow. Use all of the buttercream.

Gently roll the clingfilm around the buttercream, forming a tube.

Trim excess clingfilm from the ends of the tube, then cut it in half. Place half of the tube inside a larger piping bag fitted with a star piping tip. (You can use the entire tube at once, but I find it easier to handle when cut into two smaller pieces.)

Assembly:

Pair the cookies by size. Each pair should have one solid and one cutout heart.

Pipe a generous dollop of buttercream on the solid sides of the heart cookies.

Top with the cutout heart cookies to form a sandwich.

Enjoy immediately. Store leftovers at room temperature for up to 3 days.