To give a strong coffee flavor to your cake you can add brewed coffee, instant coffee, espresso, coffee liquor, coffee extracts, and coffee flavorings. The amount and type depends on your personal taste. Because baking requires careful measurements the best way to add coffee flavor to your cake is to find a recipe that includes one of the above mentioned coffee flavors and follow it carefully. You can also try adjusting the recipe the next time you make it or find a different recipe.