To prepare cake, lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, 3/4 cup sugar, and cinnamon, stirring with a whisk, and set aside. Combine espresso and hot water, stirring until espresso dissolves; set aside.
Place egg whites in a large bowl; beat with a mixer at high speed until foamy. Add cream of tartar and 1/4 teaspoon salt; beat until soft peaks form. Add 3/4 cup sugar, 2 tablespoons at a time, beating until stiff peaks form.
Beat in espresso mixture and vanilla extract. Sift 1/4 cup flour mixture over egg white mixture; fold in. Repeat procedure with remaining flour mixture, 1/4 cup at a time.
Spoon batter into an ungreased 10-inch tube pan, spreading evenly. Break air pockets by cutting through batter with a knife. Bake at 350° for 50 minutes or until cake springs back when lightly touched. Invert pan; cool completely. Loosen cake from sides of pan using a narrow metal spatula. Invert cake onto plate.
To prepare sauce, place egg yolks in a large bowl; set aside. Combine 1/2 cup sugar, cornstarch, and 1/8 teaspoon salt in a medium saucepan. Gradually add 1/2 cup milk, stirring with a whisk until smooth. Stir in remaining milk. Scrape seeds from vanilla bean into the milk mixture; add bean to mixture. Heat milk mixture over medium heat to a boil. Remove from heat; gradually add hot milk mixture to egg yolks, stirring constantly with a whisk.
Return milk mixture to pan. Cook mixture over medium heat until thick and bubbly (about 4 minutes), stirring constantly. Remove from heat. Spoon milk mixture into a bowl; place bowl in a large ice-filled bowl for 10 minutes or until milk mixture comes to room temperature, stirring occasionally. Remove bowl from ice; stir in liqueur. Cover and chill completely.
Remove and discard vanilla bean. Serve angel food cake with sauce, and sprinkle with toasted hazelnuts.
Great taste and soft, smooth texture to the cake..almost melt in your mouth. I made it exactly according to recipe directions, but for whatever reason it didn't rise high like most Angel Food cakes do. Perhaps I didn't beat the eggs enough or cream of tartar was bad? Nonetheless, it still came out well and husband devoured it. Will try again.
I made the angel food cake without the custard sauce and it was fantastic. The coffee flavoring is really good, but not overpowering. I served it with whipped topping and a chocolate coffee bean. My family loved it and said it was way better than a store-bought angel food cake!