1. Preheat oven to 325. Whisk together flour,
powdered sugar, cornstarch and salt in a medium
2. Beat together butter, oil, and granulated
sugar in a large bowl with an electric mixer on
medium speed until well blended. About 3
minutes. Beat in vanilla. Add flour mixture;
beat on low just until combined.
3. Pat or roll dough into a 10 and 1/2 inch
square on a baking sheet lined with parchment
paper. Bake at 325 until set and lightly
browned around the edges, about 30 minutes.
Remove from oven and immediately cut into 36
squares (a pizza cutter and ruler make this go
quickly). Cool cookies completely on baking
sheet (about 1 hour).
4. Place chocoloate in a small microwave bowl
and melt on high about 1 minutes, stirring after
30 seconds. Using a spoon or spatula, spread
chocolate over 1/2 of top side of cookie and
sprinkle sea salt over chocholate. Let stand
until chocolate sets (about and hour).