To make the sweetened condensed milk - Mix the milk and sugar in a heavy bottomed sauce pan. Keep stirring until you can see a little steam coming off the milk. Reduce the heat and keep stirring until the sugar dissolves. Turn the heat down as low as it will go. Keep an eye on the pan until you are sure that your stove can heat the milk without scorching it. Let the mixture reduce until it is a little over two cups and very thick. This will take about 2 hours. Take the pan off the stove and stir in the budder and butter. Add the vanilla and put the milk into a mixing bowl. To make the crust - Pre heat the oven to 375. Mix the graham cracker crumbs, sugar and butter with your hands. When everything is well combined, press the mixture into a 9 inch pie pan. Bake the crust at 375 for about 20 minutes. Don't let the crust burn. Let the crust cool to room temperature before filling. Lower the oven temp to 325. To make the pie- Whisk the eggs and lime juice into the sweetened condensed milk. Pour the mixture into the cooled pie crust and put it into the oven for 15 minutes. Let the pie cool to room temperature while you combine the sour cream and the powdered sugar. When the pie is cool, spread the sour cream in a thin layer over the pie. Let the pie chill in the refrigerator. Garnish the pie with key lime slices and whipped cream.