1. Adjust oven racks to upper-middle and lower-middle positions and heat oven to 350 degrees. Line 2 baking sheets with parchment paper. Place 1 cup oats in food processor or blender and process until well ground, about 30 seconds. Transfer to large bowl and stir in flour, baking powder, baking soda, and salt.
2. With electric mixer, beat butter and brown sugar together on medium-high speed until well blended and lumps are gone, about 1 minute. Scrape down sides of bowl. Add egg and vanilla and beat until light and fluffy, about 30 seconds. Add melted chocolate and beat until incorporated, about 20 seconds. Add flour mixture and beat on low speed until just incorporated, about 15 seconds. Add remaining 3/4 cup oats, chocolate chips, and nuts and beat on low speed until evenly distributed, about 10 seconds.
3. Roll 2 tablespoons of dough into balls and space 2 inches apart on baking sheets. Bake, rotating and switching position of pans halfway through baking time, until cookies are cracked and set on top but look moist within cracks, 14 to 18 minutes. Cool cookies on baking sheets on wire racks for 5 minutes, then transfer to wire rack to cool completely. (Cookies can be stored in airtight container for up to 5 days.).