1. Prepare crust: Preheat oven to 425°. Roll piecrust into a 13-inch circle on a lightly floured surface. Fit into a 9-inch pie plate; fold edges under, and crimp. Prick bottom and sides with a fork. Line piecrust with parchment paper; fill with pie weights or dried beans. Bake 9 minutes. Remove weights and parchment paper.
2. Whisk together egg yolk and cream; brush bottom and sides of crust with yolk mixture. Bake 6 to 8 more minutes or until crust is golden. Transfer to a wire rack, and cool. Reduce oven temperature to 350°.
3. Prepare filling: Stir together melted butter, 1 cup sugar, and next 2 ingredients in a large bowl until mixture is well blended. Add sweet potatoes and next 4 ingredients; stir until mixture is well blended. Pour sweet potato mixture into prepared piecrust. (Pie will be very full.)
4. Bake at 350° for 50 to 55 minutes or until a knife inserted in center comes out clean, shielding with aluminum foil to prevent excessive browning. Transfer pie to wire rack, and cool completely (about 1 hour).
5. Prepare meringue: Beat egg whites and next 2 ingredients at high speed with a heavy-duty electric stand mixer until foamy. Gradually add sugar, 1 Tbsp. at a time, beating until stiff peaks form.
6. Beat one-fourth of marshmallow crème into egg white mixture; repeat 3 times with remaining marshmallow crème, beating until smooth (about 1 minute). Spread over pie.
7. Bake at 400° for 6 to 7 minutes or until meringue is lightly browned.
Note: Pie can be made up to a day ahead. Prepare recipe as directed through Step 4; cover and chill up to 24 hours. Proceed as directed in Steps 5 through 7.