Preheat oven to 375 degrees.
Line cookie sheet with parchment paper or lightly grease.
Whisk together flour, cocoa, baking soda and salt. Set aside.
Cream sugars, butter and peanut butter together until light and fluffy.
Add in egg, vanilla and coffee and beat until well blended.
Gradually stir in dry ingredients.
Fold in chopped pecan.
Scoop out rounded tablespoons of cookie dough onto prepared cookie sheet. Dough is very thick.
Flatten each cookie slightly and bake for 6 minutes. Remove from the oven.
Lightly press half a marshmallow, cut side down, into the top of each cookie. Bake for another 1 to 2 minutes.
Remove from oven, let cool for a few minutes before transferring to a cooling rack to cool completely.