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

In a large mixing bowl, whisk flour, baking powder, and kosher salt.

In a separate bowl or the bowl of a stand mixer, add the butter and granulated sugar and mix on high speed until light and fluffy. Mix in the eggs, followed by the vanilla bean paste.

Add the dry ingredients and mix on low speed until just combined.

Divide the dough into three equal portions. Wrap two of the portions in clingfilm/plastic wrap and transfer to the refrigerator to chill.

Divide the third portion into six smaller balls. Add a few drops of gel food coloring to each, creating red, orange, yellow, green, blue, and purple doughs.

With a sharp chef’s knife, chop all of the colored doughs into small pieces. (If desired, you can also use your hands to break the dough apart.) You can make the pieces as large or as small as you’d like, and don’t worry if they’re all a little bit different—that’s fine!

On a sheet of baking parchment, arrange the rainbow dough pieces to create a mosaic pattern. Use your fingers to push the pieces together, making sure there are no gaps or holes in the dough. Top with a second sheet of baking parchment, then use a rolling pin to roll the rainbow dough to 1/4-inch thickness.

Lifting by the edges of the parchment, gently transfer the rolled dough to a tray and refrigerate for at least one hour, or until firm.

Once chilled, peel away the top layer of parchment. Use a 1 1/2-inch heart-shaped cutter to cut the rainbow dough. Stack the hearts on top of one another, then place the stack in the freezer for at least 15 minutes, or until very firm.*

While stacks freeze, remove the remaining two portions of white dough from the refrigerator and allow it to soften slightly.

Remove the stack of heart-shaped dough from the freezer. Wedge bits of white dough into the crevasses of the heart shape.

Wrap the remaining dough around the stack to form a log. Roll the log in rainbow sprinkles or rainbow nonpareils.**

Wrap the log in clingfilm/plastic wrap and freeze, at least one hour or until firm.

Once firm, use a sharp chef’s knife to slice the log into 1/4-inch thick cookies. Transfer to prepared tray. Return tray to the freezer and freeze for 15 minutes. (Alternately, you can layer the slices between squares of parchment and freeze the stacks, if your trays do not fit in the freezer.) Meanwhile, preheat the oven to 325° F / 163° C.

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

Store slice and bake rainbow heart cookies in an airtight container at room temperature for up to five days.