In a 5 or 6 Quart Bowl of a stand mixer cream butter and shortening on high speed for 3-5 minutes until smooth and pale yellow in color.
While butter and shortening are mixing measure out your cream and milk. Add your vanilla extract to milk and cream. Set aside.
Add 1 pound of confectioner sugar to your mixing bowl with butter and shortening. Top with half of the milk mixture. **(Drape a clean dish towel over the mixer and hold down against the sides of the bowl. Otherwise you'll have a white cloud of sugar everywhere!) Turn mixer on low for 1 minute. Once sugar is mixed in remove towel and scrape sides of mixer and and add remaining sugar and milk. Place towel back on mixer. Mix on low for 1 minute then once sugar is incorporated turn on high for 2 minutes. Scrape sides of mixer and mix on high for another minute.
This recipe makes a large batch. Enough to frost one 10 or 12 inch cake. You will need at least a 5 quart bowl but 6 quart is ideal, to fit the ingredients in. If you have a smaller mixer you'll need to make it in two separate half batches. *I use a Kitchen Aid 5 quart bowl but it is filled to the top.