• In a bowl, mix together the flour, baking powder, and salt. Set aside.
• In the bowl of a stand mixer, add the sugar and the zest. Rub the zest into the sugar with your fingers until well combined (this prevents the zest from clumping up in one spot and infuses the sugar with the grapefruit flavor). Fit the mixer with paddle attachment and add the butter. Cream together the butter and grapefruit sugar on medium-high speed until light and fluffy, 2-3 minutes. Incorporate the egg yolks. Reduce the speed to low and add the flour in three batches, alternating with the grapefruit juice (flour, juice, flour, juice, flour) until the dough just comes together. My dough was slightly crumbly, but held together when squeezed. Pat the dough into a disc and wrap in plastic wrap. Refrigerate 30 minutes.
• Preheat oven to 350. Prepare two baking sheets with parchment paper.
• On a lightly floured work surface, roll out the dough to 1/8 inch thickness. Using a small cookie cutter (I used a 2-inch heart cutter), cut as many cookies as you can and place them on the prepared baking sheets. Bake the cookies 9-11 minutes, or until golden on the edges and set. Allow to cool on the pan for 2 minutes, then transfer to a cooling rack to cool completely before assembling.

To Make Filling:

• In the bowl of the stand-mixer fitted with the whisk attachment, beat together the butter, powdered sugar and zest until fluffy, 3-5 minutes. Add the grapefruit juice and the food coloring and continue beating at medium-high speed until creamy and fluffy, another 1-2 minutes. Transfer the filling to a piping bag and pipe some frosting onto the under side of half of the cookies. Press an unfrosted cookie firmly on top of each frosted cookie to form a sandwich.
• Store cookies in an airtight container.