Dough needs chilling, so don't preheat quite yet.

Combine the flour, baking powder, baking soda and salt and stir well. Set aside.

In a mixing bowl, beat the butter until creamy. Add the sugars and beat, scraping bowl often, for about 2 minutes. Add the egg, egg yolk and vanilla and beat until light and fluffy.

By hand or using lowest speed of a stand mixer paddle, stir in the flour mixture until incorporated.

Stir in the oats, then stir in the Heath bars and chocolate chips.

Divide the dough into 20 equal portions onto a couple of foil lined plates or a baking sheet that fits into your refrigerator.

Chill the unbaked mounds of dough for about an hour.

When ready to bake, preheat oven to 325. Arrange dough balls on parchment lined baking sheets spacing about 2 ½ inches apart and bake for about 18 minutes (start checking at 15) or until they appear just set and edges are brown. The centers will be quite pale and may seem underbaked, but they will cook internally as they sit.

Let cool on baking sheet for about 8 minutes, then carefully transfer to a wire rack where they can finish cooling. Alternatively, you may divide each 1/20th in half and make 40 smaller cookies. If making the smaller size cookies, start checking at 12 minute. The edges tend to brown a lot quicker than the centers.