In a microwavable safe bowl, melt the butter and 1/2 cup of semi-sweet chocolate in 30 second increments until fully melted.
While the butter and chocolate are cooling. In a separate bowl, sift together the cocoa powder, flour, salt, baking soda, and sugar. Mix to combine.
Once the butter/chocolate mixture has cooled, mix in the eggs, sour cream, milk and vanilla extract.
Add the wet ingredients to the bowl with the dry ingredients, fold in 1/4 cup chocolate chips. Stir until fully combined but being careful to no overmix.
Line a muffin tin lined with muffin liners or spray with non stick baking spray. Scoop the batter into each slot then top with the remaining chocolate chips.
Bake at 400 degrees Fahrenheit for 5 minutes then drop the temperature down to 350 degrees. Continue to bake for 13-15 minutes or until a toothpick comes out clean.