Prepare cake layers; heat oven to 350 degrees.
Grease and flour two 9" round cake pans.
In large bowl with electric mixer at medium speed, beat 1/4 cup butter, 1 cup sugar, eggs, 2/3 cup milk, 1 tsp vanilla, baking powder, and flour about 4 minutes until thoroughly blended and smooth.
Spoon batter into prepared pans; bake 30 minutes until wooden pick inserted in center of each layer comes out clean.
Remove layers to wire racks to cool completely.
In 2 qt saucepan, stir 2/3 cup sugar, cornstarch and salt until thoroughly mixed; slowly stir in 2 1/2 cups milk until smooth.
Bring to boil over medium heat, stirring constantly; cook 1 minute until mixture boils rapidly and thickens.
Remove from heat; very slowly pour lightly beaten egg yolks into hot mixture, stirring rapidly and constantly to blend and keep smooth.
Return mixture to low heat; cook 1 minute longer.
Do not allow to boil.
Remove from heat; stir in 1 tbsp vanilla.
Cool completely, stirring frequently.
In small heavy saucepan over very low heat, melt chocolate and 1 tbsp butter, stirring frequently until blended and smooth.
Remove from heat; stir in confectioners sugar and 1/4 cup milk until blended and smooth.
Keep warm, covered.
Using sharp serrated knife, carefully cut cooled layers in half horizontally.
Place one layer, cut side up, on serving platter; spread with 1/3 cooled custard.
Repeat with remaining cake layers and custard, ending with cake layer, cut side down.
Spoon warm glaze over top of cake, letting mixture drip down sides.
