Melt the unsalted butter, then mix it with graham cracker crumbs and sugar in a bowl. Refrigerate it until ready to sprinkle over the muffins.

Set your oven temperature to 425°F. Line a muffin tin with paper liners and spray lightly with baking spray.

In a large bowl, whisk together the all-purpose flour, cocoa powder, granulated sugar, baking powder, baking soda, and kosher salt. Fold in the semi-sweet chocolate chunks.

In a separate bowl, melt the butter and chocolate together; whisk in the vegetable oil, whole milk, Greek yogurt, eggs, and vanilla extract.

Gently fold the dry ingredients into the wet mixture until just combined; fold in the frozen mini marshmallows.

Using an ice cream scoop, fill each muffin cup to the top. Top with extra marshmallows and chocolate chunks.

Generously sprinkle the prepared streusel over the muffin batter.

Bake at 425°F for 7 minutes, then reduce to 350°F and bake for an additional 13 minutes until a toothpick comes out clean.

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