Add butter, sugar and lemon zest - beat till fluffy. Add lemon juice and egg yolk - mix to combine.
Add flour, cornstarch and salt fold till combined.
Divide dough into 2. Set each piece onto parchment or cling wrap. fold over and roll into a cylinder about 1-1/2 inches. Chill dough at least 2 hours.
When ready to bake heat oven to 350 deg F.
Slice to 1/4 inch. Bake for 12 to 17 mins rotating pans. Golden around edges. Dust with sugar. Store at romm temp in an air tight container.