Preheat oven to 350° Fahrenheit. Position oven racks so that casserole dish can sit in center of oven. Lightly grease bottom and sides of casserole dish with neutral cooking spray and set aside.
Add ¼ cup unsalted butter to small microwave-safe bowl. Microwave butter in 15-second increments, stirring well between each, until butter is melted but not burnt. Carefully remove bowl from microwave.
Add ¾ packed cup brown sugar, ¼ cup all-purpose flour, ¾ cup coarsely chopped pecans, and melted butter to medium mixing bowl. Stir until ingredients are incorporated into crumbly mixture. Set bowl aside.¼ cup unsalted butter
Crack 1 large egg directly into large mixing bowl. Whisk vigorously until egg is well-beaten and no longer streaky.
Again add ¼ cup unsalted butter to small microwave-safe bowl. Microwave butter in 15-second increments, stirring well between each, until butter is melted but not burnt. Carefully remove bowl from microwave and let butter cool slightly.
Add melted butter, ¾ cup granulated sugar, ¼ teaspoon salt, ¼ teaspoon pure vanilla extract, and 2 cups mashed sweet potatoes to large bowl with beaten egg. Stir all ingredients together until fully combined into smooth mixture. See Notes for hand mixer option.
Transfer sweet potato mixture to prepared casserole dish. Spread mixture out into one even layer, covering bottom of dish completely.
Sprinkle praline crumble over top of sweet potato layer, distributing crumble as evenly as possible.
Place casserole dish in preheated oven. Bake casserole 30 minutes or until sweet potato layer is hot and praline crumble is browned. Note: if praline crumble browns too quickly, cover casserole dish loosely with aluminum foil.
When casserole is ready, carefully remove casserole dish from oven. Set aside to rest 10 to 20 minutes, then serve warm.