For the sweet potatoes: Line a baking sheet with foil. Line an 8-inch square baking dish with foil.
Prick the sweet potatoes all over, put them on the prepared baking sheet and roast until soft, 1 hour to 1 hour 15 minutes. Let cool slightly.
Put the butter and rosemary in a small saucepan over medium heat and cook until the butter starts to brown. Remove from the heat and discard the rosemary. Scoop the sweet potato flesh out of the skins and into a food processor. Add the butter, heavy cream, sugar, cinnamon, egg, 1 teaspoon salt and 1/4 teaspoon pepper and process until smooth.
Spread the sweet potato mixture in the prepared 8-inch baking dish.
For the crumble: Put the bacon in a medium skillet over medium heat and cook until crisp, about 4 minutes. Transfer to a paper-towel-lined plate with a slotted spoon, reserving the fat. Whisk together the flour, pecans, sugar and a pinch of salt in a medium bowl. Using your fingers, rub the butter and 2 tablespoons bacon fat into the flour mixture until coarse crumbs form. Add the bacon and toss to combine. Sprinkle the crumble over the sweet potato mixture.
Bake until lightly browned and hot throughout, about 30 minutes.
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