When dealing with food:
Al is used for flavorings= il gelato al cioccolato Chocolate (flavored) ice cream
Di is used for composition=gelato de soia ice cream made of soy (instead of with milk/cream)
So in our instance torta dolce doesn't need either. Dolce is neither the composition or the flavor (it can be a chocolate, strawberry or any other flavored tart/cake/pie) of the tart/cake/pie. It's what it tastes like.
My question was CLEARLY type what you hear, which I did, correctly, but it was wrong because it came up as the wrong TRANSLATION. Does this happen to others? It's not the first time and is so frustrating. I click on the 'my answer should have been accepted' but never get any feedback...