Line a large baking sheet with parchment paper. Place the butter and icing sugar in a large bowl and beat with an electric mixer until light and well combined. Beat in the egg yolk and lemon zest and juice. Beat in the flour until just combined. Roll the dough into 1 1/2-inch balls and place them 2-inches apart on the baking sheet. Flatten each ball into a thick disc. Chill the cookies in the fridge 20 minutes.
Set the oven rack in the middle of the oven and then preheat to 350 degress F. Bake the cookies for 13-15 minutes, or until very lightly golden. Cool the cookies on a baking rack. Place the lemon juice for the glaze in a small bowl. Slowly mix in the 6 Tbsp. of icing sugar. Drizzle the glaze over the cookies and allow to set before serving.