Preheat oven to 450 degrees.
Place butter into large mixing bowl. Peel skins off yams while still hot from cooking and place yams into bowl with butter. Using a mixer, blend butter into sweet potatoes until butter melts. Stir in sugar, nutmeg and cinnamon, mix well.
In another bowl place eggs and whisk until blended. Add the milk and vanilla and whisk to combine. Pour this mixture into potato mixture and blend well. Pour into pie shells.
Place pies on cookie sheet for better control in getting in and out of the oven. Bake for 10 minutes in preheated 450 degree oven, reduce heat to 350 degrees and continue baking for 45 additional minutes. Cool on wire racks. Makes 2 or 4 pies. Enjoy!