In a bowl of standing mixer fitted with paddle attachment, or using a hand held mixer beat butter and sugar together until light and fluffy. About 2 minutes.
Add in egg and beat well to combine. Next blend in sour cream and both extracts.
Slowly incorporate in flour, baking powder, baking soda, salt and cornstarch, then allow to mix till the flour disappears into the dough.
Divide dough into 2 equal balls and place each portions on a plastic wrap and shape them into 5-6 inches round discs. Place them in refrigerator for 2 hours or until they are firm enough to be rolled.
Preheat oven to 350 deg F. Quick Tip: When rolling, I like to roll them between to sheets of parchment paper to avoid dough sticking. Roll each disc to 1/4 inch thickness and cut into triangles.
To finish the third side of watermelon cookie slices use a large round cookie cutter or any circular lid.
Transfer cookie slices to 2 baking sheets lined with parchment paper and bake for 10-11 minutes. The cookies will be puffed up at this moment and will be just set.
Allow to cool on baking sheet for 5-7 minutes and then transfer on wire rack to complete Cooling.