Begin by melting 3 tablespoons of unsalted butter in a small bowl. In a separate bowl, combine 3/4 cup of graham cracker crumbs and 1.5 tablespoons of sugar. Pour the melted butter over the crumbs and mix until evenly coated. Transfer this mixture to the refrigerator to chill while you prepare the muffin batter.

Preheat your oven to 425°F (220°C). Line a muffin tin with paper liners and lightly coat with non-stick spray.

In a large mixing bowl, whisk together 2 cups of all-purpose flour, 1/3 cup of cocoa powder, 1/2 cup of sugar, 2 teaspoons of baking powder, 1/2 teaspoon of baking soda, and 1/2 teaspoon of kosher salt. Gently fold in 1/2 cup of semi-sweet chocolate chunks.

In another bowl, melt 3 tablespoons of unsalted butter and additional chocolate until smooth. Stir in 1/2 cup of vegetable oil, 1/2 cup of whole milk, 1/2 cup of Greek yogurt, 2 large eggs, and 1 tablespoon of vanilla extract until well blended.

Pour the wet ingredients into the dry ingredients, gently folding them together to create a batter. Fold in 1 cup of mini marshmallows.

Using an ice cream scoop, fill each muffin cup to the brim. Top with extra chocolate chunks and mini marshmallows, then sprinkle the chilled graham cracker streusel on top.

Bake in the preheated oven for 7 minutes, then reduce to 350°F (175°C) and continue baking for an additional 13 minutes until a toothpick comes out clean.

Let the muffins cool in the tin for 5 to 10 minutes before transferring to a wire rack to cool completely.