Preheat the oven to 350, and line an 8×8 inch baking pan with parchment paper.
In a bowl, combine the sugar and lemon zest, then use your hands to rub the zest into the sugar to infuse the lemon flavor into the sugar. The sugar should look like damp sand.
Whisk in the flour, salt and baking powder.
Add the lemon juice, eggs, melted butter and vanilla extract, and whisk until smooth. Stir in Loacker Wafer Classic Lemon Cookies. Pour the batter into the prepared pan.
Bake the lemon bars on the center oven rack for about 25 minutes, just until set and gently puffed. Set aside to cool completely before cutting into squares and frosting.
With an electric mixer, beat the butter and lemon zest for 1 minute until smooth. Add the powdered sugar, lemon extract and vanilla extract, beating for several minutes until light and fluffy. If needed, add 1 tablespoon of milk or cream for desired consistency.
Remove the bars from the pan by lifting the parchment paper up, then gently peeling the paper off the bars and setting them on a cutting board.
Spread the buttercream onto the lemon bars and cut into squares, or cut into squares first, and pipe the buttercream onto the individual squares.