In a large bowl, cream butter, peanut butter, and 6 tablespoons sugar until light and fluffy.
Add egg, egg yolk, and vanilla one at a time and beating well after each addition.
In a medium bowl, whisk together all-purpose flour, masa harina, cocoa powder, cinnamon, and salt.
Add dry ingredients to creamed mixture and beat until combined and dry ingredients are moistened.
Stir in peanuts.
Cover dough with plastic wrap. Chill dough at least 2 hours or overnight.
Heat oven to 350°F. Line baking sheets with parchment paper or non-stick liners.
Place remaining sugar on a small plate or in a shallow bowl.
Scoop 1 heaping tablespoon of dough. Form into a ball. Roll ball in sugar to coat. Place ball on baking sheet.
Repeat with remaining dough and placing balls 2 inches apart on baking sheets.
Use the bottom of a 1-cup measure or glass to flatten balls to 2-inch rounds. You may need to dip the measure or glass in sugar between each to keep the dough from sticking to it.
Bake 10 to 12 minutes, until middle of cookie is set.
Cool cookies on baking sheet for 1 minutes. Transfer cookies to a wire rack to cool completely.