In a medium-sized bowl, use a mixer to whip together the mashed potatoes and eggs until completely blended.
Stir in the sugar and milk.
Pour this mixture into a pie plate or an 8x8 pan that has been sprayed with cooking spray.
In a small bowl, mix together the brown sugar, flour and pecans.
Cut the butter into small chunks.
Use a pastry blender to cut the butter into the sugar mixture until it is coarse crumbs.
Mix the vanilla into the streusel topping.
Sprinkle the streusel over the sweet potato mixture.
Bake for 35 minutes or until the top is golden brown.
To prepare the sweet potatoes: Peel the potatoes and cut them into chunks. Boil them in water over medium heat until they are soft (about 10-15 minutes depending on the size of the sweet potato chunks). Drain off the water, then mash them with a potato masher or a mixer. You can also use canned sweet potatoes if you'd like. Drain the liquid off and mash. *Instead of evaporated milk, you can also use a high fat milk or cream (whole milk, half & half, or even heavy whipping cream for a rich flavor). ***You can double this recipe and bake it in a 9x13 pan. This will most likely take at least 10 minutes longer to bake.