Preheat the oven to 350 F and line two baking sheets with parchment paper.
Beat the butter, brown sugar, and granulated sugar until creamy. Mix in the egg and vanilla until smooth.
Whisk the flour, cocoa powder, baking soda, and salt in a separate bowl. Add the dry ingredients to the butter mixture and mix just until combined.
Fold the chocolate chunks, almonds, and half of the mini marshmallows into the dough. Divide the dough into 12 portions and arrange them at least 2 inches apart on the baking sheets.
Bake for 8 minutes, then press the remaining marshmallows onto the tops of the cookies. Continue baking for 3 to 4 minutes, until the edges are set and the marshmallows are puffed.
Let the cookies cool on the baking sheets for 10 minutes before transferring them to a cooling rack. The centers will remain soft and fudgy as they cool.