Preheat oven to 375° F. Line a sheet pan with parchment paper or a silicone baking mat.
In a food processor or blender, blend 1 ¼ cup of the oats until finely ground. Add the cornstarch, baking powder and salt and blend until combined.
In the bowl of a stand mixer with a paddle attachment, or in a large bowl with a hand-held mixer, beat the butter and sugar on high speed until pale and creamy, about 3 minutes. Scrape down bowl.
Add the egg and vanilla, mixing after each. Add the oat mixture and mix on low until just combined. Add the remaining 1 cup oats and the chopped chocolate and briefly mix until incorporated.
Drop by rounded tablespoons onto the prepared sheet pan. Bake until tops just begin to turn golden, about 12-14 minutes. Let cool on the pan 5 minutes, then transfer to a wire rack to cool completely.
Store cookies in an airtight container at room temperature for up to 5 days.
Calories: 163kcal, Carbohydrates: 16g, Protein: 2g, Fat: 10g, Saturated Fat: 6g, Cholesterol: 23mg, Sodium: 73mg, Potassium: 126mg, Fiber: 2g, Sugar: 6g, Vitamin A: 175IU, Calcium: 23mg, Iron: 2mg