In a medium bowl, stir together the oat flour, cornstarch, baking soda and salt.
In a large bowl, using an electric hand mixer, beat the butter and both sugars on high speed until fluffy.
In a separate, small bowl, whisk together the water, oil and baking powder until frothy and combined. Add it into the butter mixture, along with the vanilla, and beat until fluffy and combined, about 1-2 minutes on high-speed.
Add the dry ingredients into the wet and stir until combined, and a sticky dough forms. Stir in the chocolate chips.
Cover and refrigerate for at least 2 hours, up to overnight. DO NOT SKIP THIS STEP.
Once chilled, heat your oven to 325 degrees and line a baking sheet with parchment paper.
Scoop the dough by heaping tablespoons (I used a heaping cookie scoop) and roll into balls, press out very slightly - you want the balls higher than they are round. **
Bake until the tops JUST feel set and the edges are very slightly golden, about 13-15 minutes. Let cool on the pan for 5 minutes and then transfer to a wire rack to cool completely. ***