Preheat oven to 325 degrees F (165 degrees C). Line a 8x8-inch baking pan with greased parchment paper.
Mix sugar, melted butter, chocolate drink mix, dark chocolate powder, and sea salt together in a bowl. Beat eggs and vanilla extract together until smooth. Fold egg mixture into cocoa mixture, incorporating as little air as possible. Sift in flour slowly while stirring until batter is thick and shiny.
Melt chocolate in a microwave-safe glass or ceramic bowl in 15-second intervals, stirring after each melting, 1 to 3 minutes.
Pour half of the batter into the prepared baking pan; pour in half of the melted chocolate. Pour in remaining batter; top with remaining melted chocolate. Press chocolate into the batter with a spatula so that only a small amount of melted chocolate is visible.
Bake in preheated oven until a toothpick inserted into the center comes out clean, 25 to 35 minutes. Remove from oven; top with marshmallows and caramel sauce. Bake until marshmallows are melted and browned, 3 to 5 minutes. Remove from oven; top with walnuts. Cool until brownies are set, about 15 minutes.