Preheat oven to 350 degrees. On a lightly floured surface, roll piecrust into a 12 inch circle. Press into bottom and up sides of a 9-inch deep-dish pie plate; prick bottom with a fork. Fold edges under and crimp as desired. Place a piece of parchment on bottom of piecrust, letting ends extend over edges. Fill with pie weights.
Bake until light golden, approximately 15 minutes. Carefully remove pie weights and parchment. Let cool completely.
In a large bowl, whisk together sweet potato, brown sugar, eggs, melted butter, molasses, flour, vanilla, salt, cinnamon, and zest. Slowly stir in evaporated milk. Pour mixture into cooled piecrust. Bake just until center is set, 30 to 40 minutes. Cover crust rim with aluminum foil to prevent excess browning of crust. Let cool on a wire rack.
Pour hot Gooey Pecan Topping over sweet potato layer, spreading to cover edges of pie. Refrigerate at least 4 hours before serving.
Gooey Pecan Topping -- makes 2 cups.
1 1/2 cup firmly packed light brown sugar.
6 Tbs unsalted butter, melted.
4 Tbs unsulfured molasses.
2 Tbs heavy whipping cream.
2 tsp vanilla extract.
2 cups chopped toasted pecans.
1/4 tsp kosher salt.
In a small saucepan, heat brown sugar, melted butter and molasses over medium heat, stirring until sugar dissolves and mixture just begins to boil. Add cream and vanilla, stirring constantly until incorporated. Stir in pecans and salt; bring to a boil, stirring constantly. Remove from heat.