Place semi-sweet chocolate chips and butter in a microwave safe bowl.
Microwave on medium (50% power) for 1 minute, and stir; microwave at additional 30 second intervals, stirring each time, until chocolate is melted, but do not scorch (cooking time may vary with different ovens, but about 3 minutes total).
Allow chocolate to cool about 10 minutes.
Stir chocolate together with buttermilk, sugar, egg, and vanilla.
Separately, sift together the flour, baking soda, and salt.
Add the chocolate mixture to the center of the flour mix, stirring just until combined.
Fold in the milk chocolate chips.
Place batter into a greased 12-cup muffin tin 2/3 full.
Bake 20-23 minutes or until a toothpick can be removed clean.
Cool for 5 minutes in the pan, then finish cooling on a wire rack.