Preheat oven to 375 degrees F. Grease a 12-cup muffin tin or prep muffin tin with cupcake/muffin wrappers.
Melt ½ cup butter in a microwave. Allow to cool slightly at room temperature.
In a stand mixer mixing bowl, mix sugar, melted butter, buttermilk, and vanilla on low speed until just combined.
In another medium bowl, sift together flour, baking soda, and salt.
Keep mixer on low and slowly add in dry mixture, ½ cup at a time, until combined.
With a spatula, gently fold in blueberries, raspberries, and chocolate chips into the mix. Be careful not to overmix.
Use a large cookie or ice cream scoop to school muffin mix evenly divided into each greased muffin tin or cupcake/muffin wrapper. Top with handful of streusel topping (see below). Then add 4-6 chocolate chips or chocolate chunks on top of each muffin.
Make the Streusel Topping
In a medium bowl, mix together the flour, both sugars, cinnamon, and cubed cold butter using your hands or two forks. Mix together until the mixture is crumbly.
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