Preheat oven to 325°. Whisk together first 3 ingredients in a bowl.
Beat egg yolks with a handheld mixer in a medium bowl at high speed 4 to 5 minutes or until yolks become pale and ribbons form on surface of mixture when beater is lifted. Gradually whisk in sweetened condensed milk mixture, and whisk until thoroughly combined; whisk in buttermilk. Pour mixture into prepared crust.
Bake at 325° for 20 to 25 minutes or until set around edges. (Pie will be slightly jiggly.) Cool on a wire rack 1 hour. Cover pie with lightly greased (with cooking spray) plastic wrap, and freeze 4 to 6 hours. Top with Lemon-Blueberry Topping just before serving, if desired.
Strawberry-Lemon-Buttermilk Icebox Pie with Baked Gingersnap Crust: Prepare Lemon-Buttermilk Icebox Pie as directed, substituting baked Gingersnap Crust for Graham Cracker Crust and whisking 1/2 cup strawberry jam into condensed milk mixture. Microwave 3 Tbsp. strawberry jam in a medium-size microwave-safe bowl at HIGH 20 seconds. Stir 16 oz. sliced strawberries into jam. Top pie with strawberry mixture just before serving. Hands-on 25 min.; Total 6 hours, 25 min., including crust
Key Lime-Buttermilk Icebox Pie with Baked Buttery Cracker Crust: Prepare Lemon-Buttermilk Icebox Pie as directed, substituting baked Buttery Cracker Crust for Graham Cracker Crust and Key lime juice for lemon juice. Top with Sweetened Whipped Cream. Hands-on 20 min.; Total 6 hours, 20 min., including crust and whipped cream
Mango-Lemon-Buttermilk Icebox Pie with Baked Saltine Cracker Crust: Bring 1/2 cup water to a boil over medium-high heat in a saucepan. Stir in 1 cup chopped fresh or thawed frozen mango. Remove from heat, and stir in 1 Tbsp. fresh lemon juice. Process mixture in a blender until smooth. Prepare Lemon-Buttermilk Icebox Pie as directed, substituting baked Saltine Cracker Crust for Graham Cracker Crust and 3/4 cup mango puree and 1/4 cup lemon juice for 1/2 cup lemon juice. Discard remaining puree. Whisk 2 Tbsp. mango schnapps into condensed milk mixture along with puree and lemon juice. Top with Sweetened Whipped Cream and chopped fresh mango just before serving. Hands-on 30 min.; Total 6 hours, 25 min., including crust and whipped cream