Use the cacao content specified in the recipe. If the percentage is not indicated on the label, you can evaluate it by taste comparison. If you use chocolate that is higher in chocolate components (cocoa solids and cocoa butter) and lower in sugar than what is called for, a cake will have a heavier texture and a bitter taste and a buttercream or ganache will have a stiffer texture, because in effect you are adding more chocolate and less sugar to the recipe.
Begin mixing on low speed to keep the ingredients from jumping out of the bowl, and then gradually increase the speed, as indicated in the recipe. You can also use the mixer’s pouring shield or splash guard, or cover the top of the mixer bowl with plastic wrap, until the dry ingredients are moistened. If you are using a handheld electric mixer, use a deep bowl to avoid spattering, and use a higher speed than specified for the stand mixer and a longer beating time. (Beat for 2 minutes after the dry ingredients are moistened and then 45 seconds after each addition of the egg mixture.) With both methods, it’s important to stop the mixer and scrape down the sides of the bowl to ensure that the batter on the sides gets mixed in evenly. Be sure to reach to the bottom of the bowl, especially when using the stand mixer (see BeaterBlade).