How to Make It
Preheat the oven to 350°. Line a 12-cup muffin pan with liners. Spread the walnuts on a baking sheet and toast for 7 minutes, until lightly browned. Let cool, then coarsely chop.
In a medium bowl, whisk the flour, salt, baking powder and baking soda. In a stand mixer fitted with the paddle, beat the butter with the 2/3 cup of sugar at medium speed until fluffy, about 2 minutes. Beat in the egg, then beat in the sour cream and vanilla. Scrape down the side of the bowl, then beat in the dry ingredients until just incorporated. At low speed, beat in three-fourths each of the chocolate and walnuts until mixed.
Spoon the batter into the prepared muffin cups, then scatter the remaining chocolate and walnuts on top, pressing them slightly into the batter. Sprinkle with sugar and bake for 25 minutes, until a toothpick inserted in the center comes out clean. Let the muffins cool slightly, then transfer to a rack to cool.