> 1 cup (2 sticks) salted butter, softened
> 2 cups all-purpose flour, plus more for dusting
> 3 3/4 cups confectioners' sugar, divided
> 5 tbsp low-fat milk
> 1 tsp lemon juice
> Conversation hearts, as needed
> Pink sanding sugar, as needed
> Red decorative gel, as needed
Line a baking sheet with parchment. In a large bowl, with a hand mixer or stand mixer, beat together the butter, flour, ½ cup confectioners’ sugar, and a pinch of salt. On a floured surface, roll out the dough to ¼-inch thickness. Using a 4-inch rectangle cookie cutter, cut out cookies and place on prepared baking sheet. Refrigerate 15 min.
Preheat oven to 325°F. Remove cookies from refrigerator and bake 13–15 min., until golden brown around edges. Transfer cookies to wire rack and cool completely.
Meanwhile, in a large bowl, whisk together remaining confectioners’ sugar, milk, and lemon juice (the icing will be thick). Transfer icing to a resealable plastic bag or pastry bag and snip a small corner off the bag.
Pipe some icing onto each cookie. Use a butter knife to smooth out icing. Let set 10 min., until hardened. Pipe another layer of icing onto cookies as an outline in the shape of an envelope. Use the candy hearts to “seal” the envelopes. If desired, sprinkle with the pink sanding sugar and use the red decorating gel to draw squiggly lines resembling writing on an envelope on a few cookies.