Preheat oven to 350 degrees.
Add the diced sweet potatoes to a large pot. Fill the pot with enough water to cover the potatoes, then bring to a boil. Boil for 15-20 minutes, or until fork-tender. Drain, then return to the pot and use a potato masher to mash until they are smooth.
To the pot with the mashed sweet potatoes, add the brown sugar, eggs, butter, milk, and vanilla. Stir and mash together until all ingredients are well combined.
Transfer the sweet potato mixture to a greased 9" x 9" pan and set aside while you make the pecan crumble topping.
In a large mixing bowl, add the brown sugar, cornstarch, chopped pecans, and melted butter. Mix with a spatula until combined and crumbled, then sprinkle evenly over the sweet potato mixture.
Bake in your preheated oven for 30-35 minutes, or until the pecan crumble topping is lightly golden brown on top.
Remove from the oven and place on a cooling rack for 5 minutes before serving. Enjoy!