The chocolate cake exploded in the oven because I accidentally used three cups of baking soda instead of flour.

"Because" introduces the adverb clause explaining the reason for the explosion. "Because of" is incorrect because it is followed by a full clause, not a noun.

As the kitchen ceiling is now completely covered in batter, we will be taking a short commercial break!

"As" introduces the reason for taking a break. "So" and "Therefore" indicate result, but the clause they are attached to ("the ceiling is covered...") is the reason, not the result.

Please don't change the channel, since I still need to show you how to make the vanilla frosting.

"Since" provides the reason why the viewers should stay tuned. "Therefore" is a conjunctive adverb and grammatically incorrect here, while "because of" requires a noun phrase.