In a large heavy saucepan, heat milk and sugar until bubbles form around sides of pan. Whisk a small amount of hot mixture into egg yolks. Return all to the pan, whisking constantly.
Cook and stir over low heat until mixture is thickened and coats the back of a spoon. Quickly transfer to a bowl; stir in peanut butter until blended. Place in ice water and stir for 2 minutes. Stir in cream and vanilla. Refrigerate for several hours or overnight.
Fill cylinder of ice cream freezer two-thirds full; freeze according to the manufacturerâs directions.
For cake, let egg whites stand at room temperature for 30 minutes. In a small heavy saucepan, melt chocolate and butter over medium-low heat. Remove from the heat; whisk in egg yolks.
In a large bowl, beat egg whites and cream of tartar on medium speed until soft peaks form. Gradually beat in sugar on high until stiff glossy peaks form and sugar is dissolved. Fold into chocolate mixture.
Transfer to a greased and floured 13-in. x 9-in. baking pan. Bake at 325° for 20-25 minutes or until set. Cool completely. Cut into 12 pieces.
For sauce, in a small bowl, whisk egg yolks and sugar. In a small saucepan, heat heavy cream and espresso powder until bubbles form around the sides of pan. Whisk a small amount into yolk mixture. Return all to pan, whisking constantly. Cook until thickened. Remove from the heat; stir in liqueur. Refrigerate until serving.
Serve cake with ice cream and sauce. Garnish servings with chopped chocolate. Yield: 12 servings.