Q. It is hard to get cream of any kind here [India], so in baking cakes is there a substitute I can use? Same problems in getting cooking chocolate semisweet, bittersweet, or whatever so is there a substitute for this also? Can some proportion of cocoa and/or butter do the trick?
Yes, there are some substitutions you can try for both cream and chocolate. In the case of a cake with cream, though, you might prefer to find a different recipe. There are scads of great cakes that don't include a drop of cream. And cakes tend to follow fairly traditional formulas, so that if you mess with one ingredient too much, it might upset the balance of ingredients.
But there are certainly times when cream is useful, even necessary, in cooking. For a cup of whipping cream or heavy cream, which has between 36% and 40% butterfat, use 1/3 cup of butter and about 3/4 cup of milk. For 1 cup of light cream (20% butterfat), use 3 tablespoons of butter and about 7/8 cup of milk. And for half-and-half (10% to 12% butterfat), use 1-1/2 tablespoons of butter and 3/4 cup plus 2 tablespoons of milk.
On the chocolate front, for 1 ounce of unsweetened chocolate, mix 3 tablespoons of cocoa with 1 tablespoon of butter or other fat. For 6 ounces of semisweet chocolate, mix 6 tablespoons of cocoa with 7 tablespoons of sugar and 1/4 cup of butter or other fat. For the same amount of bittersweet chocolate, simply invert the measurements of the cocoa and sugar.