Preheat oven to 325°F Grease and flour a 9x5x3-inch loaf pan; set aside. In a medium bowl stir together flour, sugar, cream of tartar, salt, and baking soda; set aside.
In a large bowl beat butter with an electric mixer on medium to high speed for 30 seconds. Add flour mixture; beat for 2 to 3 minutes or until fine crumbs form.
In a small bowl combine the water and the 1 tablespoon coffee crystals, stirring until coffee crystals are dissolved. Add coffee mixture and vanilla to butter mixture; beat on low speed until combined. Beat on medium speed for 2 minutes. Beat in eggs and melted chocolate on low speed until combined. Beat on medium speed for 1 minute.
Pour batter into prepared pan, spreading evenly. Bake for 75 to 80 minutes or until a toothpick inserted near center of cake comes out clean.
Cool in pan on a wire rack for 10 minutes. Remove from pan; cool completely on rack. If desired, top each serving with Coffee Cream, chocolate curls, and additional coffee crystals.
Coffee Cream: In a medium bowl combine 1 cup whipping cream, 1 tablespoon powdered sugar, and 1 teaspoon instant coffee crystals. Cover and chill for 10 minutes. Beat with an electric mixer on medium to high speed until stiff peaks form (tips stand straight).