In a large mixer bowl, beat butter for 30 seconds. Combine flour, sugar, salt, cream of tartar, and baking soda. Add to mixer bowl and beat with an electric mixer for 2 to 3 minutes or until mixture forms fine crumbs.
Dissolve coffee crystals in water. Add to beaten mixture, along with vanilla. Beat on low speed until combined. Beat 2 minutes more on medium speed.
Add eggs and melted chocolate. Beat on low speed until well combined. Beat 1 minute more on medium speed.
Pour into a greased and floured 9x5x3-inch loaf pan. Bake in a 325 degree F oven for 1 hour 20 minutes.
Prepare Coffee Cream, if you like.
Cool the cake in loaf pan for 10 minutes. Remove the cake from the pan. Cool on a wire rack. Top slices with Coffee Cream and shaved chocolate, if you like. Makes 12 to 16 servings.
Melt the chocolate in a microwave-safe 1-cup measure. Microcook on 100 percent power (high) for 1-1/2 to 2 minutes or until soft enough to stir smooth, stirring every minute.