In a glass bowl (over simmering water), melt the chocolate and butter together. Allow it to cool, but not harden. You can also do this in the microwave, heating in 30-second increments and stirring in between.
In a small bowl, whisk together the eggs, vanilla extract, and sugar. Add this mixture to the melted chocolate. Stir to combine.
Stir in the flour, baking powder, and salt until combined. Then stir in the chopped nuts and marshmallows. Cover and chill in the refrigerator for at least 1 hour and up to overnight.
Spoon or scoop onto a baking sheet with a baking mat or parchment paper. Bake at 325°F for 8-10 minutes. Allow to cool for a few minutes on the baking sheet then remove to a cooling rack.
The marshmallows do have a tendency to ooze out the edges after baking. Mostly this is a cosmetic issue, ha. But what I do is use the end of a spatula to push the melted marshmallow back toward the cookie while they are fresh out of the oven and still on the baking sheet. But, this isn't necessary, just a little tip you can try.