Sift flour, coco powder, baking soda, baking powder and salt together in a medium bowl. Set aside.
In a large bowl, cream the butter and both sugars together until light and fluffy, 2-3 minutes.
Add eggs one at a time, mixing well between each addition. Add vanilla and mix until combined.
Gradually add the dry ingredients, paying attention to the dough consistency. The dough should be slightly sticky when touched. You may need to add a little extra flour or leave a little bit out (see note).
Add the pecans, mini marshmallows, and chocolate chips and mix until just combined.
Drop rounded tablespoons onto parchment lined cookie sheets (or you can grease them), I usually fit a dozen per sheet. (See note for directions on how to make extra thick and chewy cookies.)
Bake in preheated oven for 8-10 minutes, rotating the sheets halfway and checking at 8 minutes. The cookies should appear done on the edges but still soft in the middle when they are finished cooking.
Let cool on the cookie trays for 5 minutes before transferring to a cooling rack.
The amount of flour you use is going to vary depending on where you live, the weather, etc. The dough for these cookies should be slightly sticky but not dry or overly gooey. You may need to adjust based on your particular situation.To get extra thick and chewy cookies, I use a scoop that is 1.5 inches in diameter. I scoop (leveled scoop) 8 cookies onto my sheet tray, very lightly press down (just enough to make the tops a bit flat), and then place another scoop on top of the first scoop. Bake until the edges are done and the centers are slightly doughy, about 10-12 minutes for this recipe. Perfect thick and chewy cookies every time!