1. Preheat oven to 350°. Coat a 9-in. round cake pan with 2-in. sides with cooking-oil spray. Mix walnuts, brown sugar, melted butter, and nutmeg and spread into pan.
2. In a large bowl, cream 1 cup granulated sugar with softened butter. Beat in 2 eggs.
3. In a medium bowl, mix flour, baking powder, baking soda, and 1/4 tsp. salt. Beat 1/3 flour mixture into butter mixture, then 1/2 cup sour cream; repeat with another 1/3 flour mixture, then remaining sour cream and flour mixture. Spoon evenly into pan.
4. Bake cake until a toothpick inserted in center comes out clean, 40 to 50 minutes. Cool in pan 10 minutes. Run a thin knife around pan edge to loosen cake; invert onto a rack. Let cool completely, then slice in half horizontally with a serrated knife.
5. In a saucepan, mix remaining granulated sugar, the cornstarch, remaining salt, and milk. Stir over medium heat until thickened, 5 minutes. Remove from heat.
6. In a small bowl, lightly beat remaining egg with about 1/3 cup hot milk mixture. Return it to milk mixture in pan and stir over medium heat until hot and very thick, 1 to 2 minutes. Stir in vanilla. Remove custard from heat and whisk often until cool, at least 20 minutes.
7. Set bottom layer of cake, cut side up, on a serving platter. Spread custard over cake, then top with remaining layer, nut side up.
Note: Nutritional analysis is per serving.