Step 1: Preheat your oven to 350°F (175°C). Grease your 9x13-inch baking pan or line it with parchment paper for easy removal.
Step 2: In a large mixing bowl, whisk together the 1/3 cup unsweetened cocoa powder, 1/2 cup vegetable or canola oil, and 1/2 cup softened butter until smooth.
Step 3: Stir in the 2 cups granulated sugar until well combined. Add the 4 large eggs one at a time, mixing well after each addition. Then, stir in 2 teaspoons vanilla extract.
Step 4: In a separate bowl, combine 1 1/2 cups all-purpose flour and 1 teaspoon salt. Gradually add this mixture to the wet ingredients, stirring until just combined.
Step 5: Pour the brownie batter into the prepared baking pan and spread it evenly. Bake in the preheated oven for about 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Step 6: Once the brownies are baked, remove them from the oven and immediately sprinkle 3 cups mini marshmallows evenly over the top. Return to the oven for an additional 5-7 minutes, or until the marshmallows are fluffy and slightly golden.
Step 7: While the brownies cool, whisk together the 1/2 cup melted butter, 1/3 cup cocoa powder, 1/3 cup evaporated milk, and 1 teaspoon vanilla extract. Gradually add 3 1/2 cups powdered sugar, mixing until smooth and creamy.
Step 8: Spread the frosting over the warm marshmallow layer, allowing it to slightly melt into the marshmallows. Let the brownies cool completely before cutting into squares.
