In a large bowl mix together melted butter, white sugar and brown sugar using a rubber or silicone spatula. Add the egg and continue to mix together until uniform consistency forms.
Add cocoa powder and then oat flour, ½ cup at a time, until well-incorporated. With the last ½ cup of oat flour also add the baking soda. Fold in chocolate chips or chunks.
Cover the bowl and refrigerate for at least one hour, up to overnight.
When ready to bake, preheat the oven to 350F.
Line a large baking sheet with parchment paper. Using an ice cream scoop, scoop the dough onto the baking sheet, about 2-3 tablespoons. Leave space between the cookies for them to expand.
Bake for 11-12 minutes until edges brown. Let cool completely on the baking sheet.
Store in an airtight container for 3-5 days. Freeze for up to 3 months.