Preheat the oven to 350°F and grease a 9x13-inch baking pan.
In a large bowl, whisk together the melted butter, sugar, and cocoa powder until combined. Whisk in the eggs one at a time, then the vanilla, until the mixture is thick and glossy.
Stir in the flour, baking powder, and salt just until combined, then fold in the pecans. Spread the batter evenly into the prepared pan.
Bake for 25 to 30 minutes, until a toothpick inserted in the center comes out with a few moist crumbs.
Scatter the mini marshmallows evenly over the hot cake and return it to the oven for 2 to 3 minutes, until the marshmallows are puffed and soft.
While the marshmallows bake, melt the butter with the cocoa powder and almond milk in a saucepan over medium heat, whisking until smooth and just simmering. Remove from the heat and whisk in the powdered sugar and vanilla until smooth.
Immediately pour the frosting over the marshmallows and spread it gently, letting it swirl into a marbled top. Sprinkle with the pecans and cool for at least 2 hours before slicing.