Depending on the thickness of your caramel sauce, the best option for drizzling is a reusable plastic squeeze bottle or piping bag with a smallish round tip. This will give you way more control than a spoon or pitcher. You've got two options for timing. In real life, for convenience, I like to drizzle the sauce onto the whole cake after removing from the fridge and before serving. If you're feeling extra-fancy, you can drizzle each slice after cutting so that the caramel runs gently down the sides. It's up to you!