1
: Preheat oven to 350°F (180°C). Grease three 8” (20 cm) round cake pans.
2
: Batter: Place 1 cup (250 mL) milk chocolate chips and water in a medium saucepan. On medium heat, melt chocolate, stirring frequently. Cool mixture.
3
: Beat butter and sugar until light and well combined. Add eggs, one at a time, beating well after each addition. Add vanilla and melted chocolate mixture.
4
: Sift flour, cocoa powder, baking soda and salt in a large bowl.
5
: Add the flour mixture and sour cream in three additions, beginning and ending with the flour mixture. Beat just until combined. Stir in remaining milk chocolate chips.
6
: Divide batter evenly in prepared baking pans. Bake for 30-35 minutes or until a toothpick inserted in centre of cakes comes out clean. Let cakes cool in pans on wire cooling rack for 15 minutes. Remove from pans and cool completely.
7
: Filling: Combine milk, sugar, butter, egg yolks, and salt in a medium saucepan. Cook, stirring constantly, over medium heat. Cook until mixture is boiling and has thickened, about 15-18 minutes. Remove from heat. Stir in chopped, toasted walnuts and vanilla. Allow mixture to cool until spreadable (about 30-60 minutes).
8
: Assembly: If cakes are domed, slice off tops to make them even. Place one layer on serving plate. Spread filling between layers and on top of cake.