• Preheat oven to 350 degrees and line cookie sheet with parchment paper or a Silpat.
• Cream sugar and butter together in a stand mixer with paddle attachment (or a large bowl with a hand mixer) until light and fluffy. Add eggs and vanilla and mix to combine.
• In a separate large bowl, whisk together flour, baking powder, and salt. Stir in ½ cup crushed conversation hearts.
• Slowly pour the flour mixture into the wet mixture, mixing on low all the time. Stop mixer when dough is just combined taking care not to over-mix.
• Place rounded tablespoons of dough on prepared cookie sheet about 1 ½ inches apart. Bake for about 14 minutes, until edges are just starting to turn golden. If you’re using the large conversation hearts, take baking sheets out of the oven after about 10 minutes, push the hearts in the center of each cookie, and return baking sheets to the oven.
• Let cookies rest on baking sheets for about 5 minutes before moving them to wire racks to cool completely.
• Once the cookies are cool, spread one with a thin layer of marshmallow fluff and then top with another cookie. Roll the cookie on its side in the bowl of remaining crushed conversation hearts.
• Place cookies in the fridge for about 10 minutes to let marshmallow fluff firm up.

Marshmallow Fluff

• In the bowl of a stand mixer with whisk attachment beat together egg whites, corn syrup, and salt on high speed for 4-5 minutes until the mixture thickens quite a bit. Your mixture will have started clear, but after beating, it will turn bright white
• Mix in the confectioners' sugar, vanilla, and cream of tartar on low speed, until combined.
• Store in an airtight container in the fridge for up to ten days.