Preheat oven to 350º F. Grease or paper-line 12 standard-sized muffin cups.
Combine flour, cocoa powder, baking soda, and salt in a large bowl. Whisk until well mixed.
In a separate bowl, whisk together buttermilk, brown sugar, oil, egg, and vanilla.
Add the wet ingredients to the dry and mix until just combined, no more than 15 to 20 strokes. Fold in chocolate chips.
Divide the batter equally between muffin cups; they will be quite full. Grease a small spoon and dip it into each mound of batter, twirling and pressing down to create wells in the centers. Fill each well with a generously rounded teaspoon of peanut butter. Swirl the peanut butter into the batter with a skewer.
Sprinkle chopped peanuts and chocolate chips over the tops of the muffins. Press the toppings gently into the batter.
Bake for 13-16 minutes, until muffins are set and a toothpick inserted in the center comes out mostly clean.
Let cool in pan for about 5 minutes before removing to a rack. Serve warm or at room temperature.