Preheat oven to 350 degrees.
Grease a 9 x 13 metal pan then line with parchment paper. Set aside.
Chop chocolate bar then microwave with butter in a large heat-safe bowl in 30 second increments, stirring after each increment until melted and smooth.
Add granulated sugar and brown sugar to bowl then whisk vigorously until completely combined.
Cool for for 5 minutes before adding eggs.
Add eggs, one at a time, whisking vigorously after each addition until fully incorporated.
Add vanilla then stir until evenly distributed. Set aside.
In a separate bowl, sift together flour, cocoa powder and salt.
Add dry ingredients to wet, stirring with a spoon until just combined. Avoid overmixing.
Pour batter into baking pan then smooth top with a spatula to level.
Bake for 25 minutes then remove brownies from the oven and top with chopped almonds, chocolate chips and mini marshmallows.
Return brownies to the oven and bake an additional 5 minutes or until marshmallows begin to puff and a toothpick inserted into the center comes out with a few moist crumbs. Avoid overbaking.
Cool brownies completely before cutting. For pristine slices, clean knife with hot water in between cuts.
Store in an airtight container at room temperature up to 3 days.