Heat the oven to 350°F and arrange a rack in the middle. Coat a 13-by-9-inch baking dish with butter; set aside.
Combine 3/4 cup of the chocolate chips and all the butter in a small saucepan. Cook over low heat, stirring constantly, until melted, about 8 minutes. Remove from heat and let cool slightly.
Whisk sugar, eggs, vanilla, and salt in a large bowl until eggs are broken up and smooth. Add chocolate mixture and stir to combine. Add flour and baking powder and stir until just incorporated. Add 3/4 cup of the marshmallows and 3/4 cup of the almonds and stir to combine.
Transfer batter to the prepared dish and spread evenly. Bake until a toothpick inserted in the center comes out mostly clean, about 25 to 30 minutes. Remove from the oven.
Move the rack to the top of the oven. Sprinkle remaining chocolate chips, marshmallows, and nuts over the surface of the batter. Return to the oven and continue baking until marshmallows are puffed and golden brown, about 10 minutes more. Remove from the oven and cool completely on a wire rack before cutting.