Preheat the oven to 400°F. Scrub sweet potatoes and pierce several times with a fork. Place on a foil-lined baking pan and bake until tender, about 1 hour. Remove from the oven and reduce oven temperature to 350°F.
When potatoes are cool enough to handle, remove and discard skins. Mash the sweet potato flesh in a large bowl until they are smooth. Add the evaporated milk, granulated sugar, vanilla, and ½ cup butter. Beat with an electric mixer or whisk by hand until well blended. Spoon potato mixture into a greased shallow 2-quart baking dish. If making ahead, stop here, cover the dish with foil, and set aside up to 2 hours until ready to bake.
When ready to bake, combine pretzel pieces, pecans, cranberries, apples, brown sugar and ½ cup butter together in a medium bowl. Sprinkle pretzel mixture evenly over the prepared sweet potato mixture. Bake, uncovered, for 25-30 minutes or until the edges are bubbly. Serve warm or at room temperature.