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 and baking powder 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. PREPARE EGG CUSTARD: 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. PREPARE CHOCOLATE GLAZE: 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. To assemble: 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.Makes 12 to 16 servings.