Preheat oven to 325°.
Coat a 12-cup Bundt pan with cooking spray. Combine cocoa and baking chocolate in a small bowl; add boiling water, stirring until chocolate melts.
Beat granulated sugar and butter with a mixer at medium speed until well blended (about 5 minutes). Add vanilla and egg whites, 1 at a time, beating well after each addition. Lightly spoon flour into dry measuring cups; level with a knife. Combine flour and next 3 ingredients in a large bowl, stirring well with a whisk. Combine buttermilk and coffee granules, stirring until granules are dissolved. Add flour mixture to sugar mixture, alternating with buttermilk mixture, beginning and ending with flour mixture. Stir in cocoa mixture. Spoon batter into prepared pan.
Bake at 325° for 1 hour or until a wooden pick inserted in center comes out clean. Cool 10 minutes; remove from pan. Cool on a wire rack.
To prepare glaze, combine melted chocolate bar and milk in a small bowl, stirring until smooth. Drizzle chocolate glaze over cake. Combine powdered sugar and milk in a small bowl, stirring well with a whisk. Drizzle glaze over cake.