1-First: mix 1/3 cup cocoa powder with 1/2 cup vegetable or canola oil in a bowl until well combined. Add 1/2 cup softened butter and stir until the mixture is smooth and creamy, creating a solid base for your batter. Next, incorporate 2 cups of sugar and blend thoroughly to dissolve it evenly.

2-Then, add 4 large eggs one at a time, mixing after each addition to keep the batter consistent. Stir in 2 teaspoons vanilla extract for that extra depth of flavor. In a separate bowl, combine 1 1/2 cups all-purpose flour and 1 teaspoon salt, then add this to the wet ingredients and mix just until everything comes together without overworking the batter.

3-Pour the batter into a greased 9×13 inch pan and bake at 350°F (175°C) for 25 to 30 minutes, or until a toothpick inserted in the center comes out clean. Once baked, sprinkle 3 cups of mini marshmallows evenly over the top and return the pan to the oven for 2 to 3 minutes until the marshmallows puff up nicely. Let the brownies cool completely on a wire rack before moving on.

4-Adding the Frosting: For the frosting, combine 1/2 cup melted butter, 1/3 cup cocoa powder, 1/3 cup evaporated milk, 1 teaspoon vanilla extract, and 2 cups powdered sugar in a bowl. Beat until smooth, then gradually add an additional 1 to 1 1/2 cups powdered sugar until you reach a spreadable consistency, typically about 3 1/4 cups total. Spread this over the cooled brownies and cut into squares for serving.