How to Make It
Prepare the Cookie: Preheat oven to 375°F. Line a 17- x 12-inch rimmed baking sheet with parchment paper. Lightly coat parchment with cooking spray.
Beat butter, brown sugar, and granulated sugar with a heavy-duty electric stand mixer on medium speed until light and fluffy, 4 to 5 minutes. Beat in egg and vanilla until smooth and incorporated.
Stir together oats, flour, salt, baking soda, and cinnamon in a large bowl until well combined. Add oat mixture to butter mixture. Beat on low speed just until combined. If desired, stir in toffee bits.
Press dough evenly into prepared baking sheet. Bake in preheated oven until light golden brown, 23 to 25 minutes. Cool in pan on wire rack 10 minutes. Remove Cookie and parchment from baking sheet, and cool completely on rack, about 20 minutes.
Prepare the Filling: Beat butter, vanilla, and salt with a heavy-duty electric stand mixer on medium speed until smooth and incorporated, 1 to 2 minutes. Add powdered sugar and zest; beat on low speed until smooth, 2 to 3 minutes.
Place Cookie, upside down, on a cutting board. Remove and discard parchment. Cut Cookie in half crosswise. Spread Filling on half of Cookie. Place unfrosted Cookie half on top, right side up. Cover with plastic wrap, and chill 1 to 2 hours. Cut into 32 bars.