Preheat oven to 350 degrees.
Butter and flour an 8x8 square baking pan.
In a medium saucepan, combine fine-quality dark chocolate and butter. Melt over low heat, stirring frequently, until completely melted and smooth. Remove from heat.
Allow mixture to cool for about 3 minutes, then add sugar and vanilla and whisk until combined.
Add eggs, one at a time, whisking well after each addition. Mixture should be smooth and glossy.
Stir in salt and flour until combined.
Stir in chocolate chips until evenly distributed. (optional)
Pour batter into pan.
Place peanut butter into a small microwave-safe bowl.
Heat for about 15 seconds, just enough time for the peanut butter to soften.
Stir peanut butter rapidly until smooth and a little runny.
Pour peanut butter evenly over surface of brownies.
With a knife, "cut" peanut butter into brownie surface to swirl it around. (make a beautiful design if you're up for it!)
Bake for 30-33 minutes, or until toothpick comes out with crumbs (not goo).
Cool brownies completely before cutting into squares.