Melt butter to make compound butter, and subsequently refrigerate

Take double cream (you want a 48% milk fat, which is hard to get in the U.S. outside of a specialty market; heavy cream has a 30-40% milk fat content) and shake it. Forever. To be more specific, after sealing your double cream in, say, a jam jar, shake it until you hear the sloshing sound of butter forming (which will take anywhere from 15 minutes to an hour); drain the liquid off and then rinse the butter until the water runs clearly over it. Squeeze your butter (I use cheese cloth or a linen napkin for this) to rid it of excess liquid (excess liquid can lead to rancidity), shape it into a block, and wrap it with wax paper.

Generally speaking, butter spreads have a water component to them. That's what the emulsifier is for, as it keeps the fat and water from separating.

The problem is that you can't fry/cook with it. If you place it in a pan it will separate, and sizzle in a very unpleasant manner. If I remember correctly, it actaully smells quite awful in the process.

As for baking, I think it'll be fine anywhere there needs to be fat as fat. Wherever you need fat as something to hold the structure, you had probably best not use it. Caveat emptor, as I don't bake very much at all.

Let it be aid, however, that margarine as opposed to butter spread, can be used anywhere that butter is used. It isn't as tasty, and has trans-fats. On the other hand, it has less cholesterol.