Combine chocolate chips and baking chocolate in a medium size microwave-safe bowl. Microwave on high for 1 minute. Stir and return to microwave for 30 second intervals, stirring in between, until chocolate is melted and smooth. This will take about 2-2 ½ minutes. Set aside to cool.

In the bowl of an electric mixer, combine butter and cream cheese. Beat for 1 minute until smooth. Add cooled chocolate mixture and beat until incorporated.

With mixer on low speed, add powdered sugar and ½ cup milk. Beat until smooth, adding more milk, as needed to achieve smooth, fluffy consistency. Beat on medium-high speed for 4 minutes. If spreading icing with a knife, you'll want a fairly loose consistency. Piping requires a somewhat stiffer consistency. You made need to add a bit more milk or powdered sugar to get it just right. If you're doing the swirly piping, try a small amount in your pastry bag to see if it is "right". It should flow easily out of the bag but create distinct swirly patterns when you pipe. You don't want it to be so loose that the pattern "wilts" or so stiff that you have to use a lot of pressure.