Preheat oven to 350 degrees F.
Over hot water in a small bowl, melt the butter and chocolate together. Let cool to almost room temperature while you prepare the rest of the recipe.
In a mixer or with a whisk, whip the eggs with the sugar until fluffy. Stir in the vanilla and salt with a wooden spoon. Add the melted chocolate mixture and stir. Add the flour and stir in only until incorporated. Add nuts and mix briefly.
Grease a non-stick mini muffin (or regular) tin and divide the batter evenly by spoonfuls filling each 3/4 full. Top each with a mini marshmallow, pressing it in slightly. Bake for 20 minutes for chewy brownies and 25 minutes for cake-ier brownie bites. Let cool then ease them out of the pan.