In a large bowl with an electric mixer or the bowl of a stand mixer with the paddle attachment, mix together the flour, sugar, brown sugar, baking powder, baking soda and salt. Add the butter, egg and vanilla and beat well until mixture comes together as a moist dough. Stir in the oatmeal until combined, then stir in the chocolate chips. Cover bowl with plastic wrap and pop into the fridge for 1 hour.
Preheat oven to 350F.
Remove dough from fridge and scoop out a rounded Tbsp of dough. Form into small balls. Dip ball into white sugar and place onto a large, ungreased cookie sheet sugar side up. Be sure to leave plenty of space between your balls (4 inches apart is best) and place them at least 2 inches in from the edge of the pan, as they will spread as they bake. If you have to bake several batches, pop the bowl with the dough back in the fridge while the first batch bakes. (I usually get 20-22 cookies)
Bake in preheated oven for 13-15 minutes, or until golden around the edges, but paler and still moist looking in the middle. For really crispy cookies, leave in the oven a few minutes longer. Remove from oven and allow to sit on the baking sheet for 1 minute, then carefully (they will be soft) remove to a cooling rack. Cookies will firm up as they cool.
Be sure to read the "Cook's Notes" in the original post, for more tips, options, substitutions and variations for this recipe!