Crusting Vanilla Buttercream Frosting

1 cup unsalted butter (2 sticks), room temperature
1 cup solid vegetable shortening (7 oz)
2 lbs. powdered sugar
1 1/2 teaspoons clear vanilla extract
3-4 Tablespoons milk, as much as you need to get the consistency you want

Cream butter and shortening in a mixing bowl.  Add the vanilla and mix again.  Slowly add in the powdered sugar, a little at a time until completely incorporated.  This will take time and probably be messy — drape a dish towel over your  mixer to contain the snowstorm.  When all of the sugar has been added, start adding one tablespoon of milk at a time to thin out the frosting to the consistency you want. Use immediately or store in the fridge in an airtight container for up to 2 weeks.  Rewhip before using.