Preheat oven to 375 Line baking sheets with parchment paper or silicone baking mat.
Melt butter in a large heavy saucepan.
Remove from heat, and stir brown sugar into melted butter.
Let stand 5 minutes.
Add egg and egg yolk; stir to combine.
Stir in vanilla extract.
Add flour, cocoa powder, baking soda, and salt. Stir until just combined.
Stir in pecans and chopped chocolate just until incorporated.
Using a medium cookie scoop (1.5 tablespoon), drop scoops of dough 2 inches apart on prepared cookie sheet.
Bake for about 6 minutes.
Gently press 3 marshmallows into the top of each cookie.3
Continue to bake for 2-3 minutes, or until the edges of the cookies are set.
Let cool on cookie sheet for 5 minutes.
Remove to wire rack to finish cooling.
Correctly measuring the ingredients is key. I suggest either weighing the ingredients or aerating the flour (sifting or fluffing) and then scooping and leveling. Packing in too much flour will cause the cookies not to spread or to be dry and crumbly.
Or scant 1/2 teaspoon table salt
Or you can let the cookies finish baking, and immediately press marshmallows into the top of the baked cookies.
I recommend stirring this recipe together by hand rather than using a hand or stand mixer.