In a blender or food processor, grind oats until fine. Transfer to a small bowl, then stir together with the whole wheat flour. Set aside.
In a large bowl, beat together the butter, brown sugar, granulated sugar, vanilla, espresso powder, cinnamon, and salt until smooth.
Beat in the baking soda, vinegar, and egg.
By hand, stir in the flour mixture, then the chocolate chips and walnuts, just until combined. For the best results, cover and chill in the refrigerator overnight.
Place rack in the center of the oven and preheat oven to 350 degrees F. Line two baking sheets with parchment paper or a silpat mat. Drop the dough by tablespoonfuls onto the cookie sheets, then bake for 9 minutes, until the bottoms of the cookies are barely beginning to brown. The cookies may appear underdone.
Remove cookies from oven, let cool on the baking sheet for 3 minutes, then transfer to a wire rack to cool completely.
Notes
TO STORE: Leftover cookies will keep in an airtight storage container at room temperature for up to 1 week. They will lose a bit of softness but still be absolutely delicious.
TO FREEZE: Freeze baked cookies in an airtight freezer-safe storage container or ziptop bag for up to 3 months. Thaw and enjoy as desired.
TO MAKE IN ADVANCE: You can prepare the cookie dough in advance and freeze it for easy cookies on demand. Prepare the cookie dough, and drop it as directed onto two parchment-lined baking sheets. Freeze until solid. Transfer the frozen cookie dough balls to an airtight freezer-safe storage container or ziptop bag. Bake directly from frozen as needed, adding a few extra minutes to the baking time.