Step 1: Line your muffin tin with muffin liners and set aside. Preheat your oven to 350°Fahrenheit. In a medium bowl, combine all of the streusel ingredients. Set aside. In a large bowl, whisk together the milk, eggs, vanilla extract, oil, and melted butter. Add the granulated sugar and whisk until smooth.
In another bowl, whisk the flour with the cocoa powder, baking powder, baking soda and salt. Slowly add the dry ingredients into the wet ingredients and mix until just combined. Stir in the chocolate chips.
Scoop the muffin batter into the prepared liners, making sure to fill until almost the top. Sprinkle the chocolate streusel on top and bake for 12-17 minutes.
A toothpick inserted should come out clean. Let the muffins cool in the pan for 10 minutes before removing to a wire cooling rack to cool completely!