Heat oven to 400°F (or 375°F for dark or nonstick cookie sheet). Line cookie sheet with cooking parchment paper. In medium bowl, stir cookie mix and 1 cup butter until soft dough forms.
On lightly floured surface, roll dough about 1/8 inch thick.
Cut into desired shapes, about 2x2 inches.
Place 1 inch apart on parchment-lined cookie sheet.
Sprinkle with sugar.
Bake 6 to 8 minutes or until edges are light golden brown. Cool 2 minutes; remove from cookie sheet to cooling rack. Cool completely, about 15 minutes.
Meanwhile, in large bowl, beat Lemon Buttercream Filling ingredients with electric mixer on medium speed until light and fluffy.
Spread about 1 heaping measuring teaspoonful filling on bottom of 1 cooled cookie. Top with second cookie, bottom side down; gently press cookies together. Store cookies in airtight container.
