Preheat oven to 350 F. Line two cookie sheets with parchment paper.
In a large mixing bowl, use electric mixers to beat together oil and peanut butters. Beat in the sugars until fluffy. Add the milk and vanilla.
Sift in 3/4 cup of the flour along with the cornstarch, baking powder and salt. Beat until smooth. Add the chocolate chips then use your hand to mix the batter the rest of the way. Sift in the remaining flour and mix until well combined.
Place about 2 tablespoons of batter on the trays, a few inches apart. Lightly flatten with your hand. Bake for about 10 minutes, until golden on the edges. Let cool on baking sheets for a few minutes, then transfer to a cooling rack to cool completely.