Chocolate (and white chocolate) is usually a mass noun, therefore it does not usually work with a. This is the same for other mass nouns such as coffee, beer.

However, if you are talking about a serving of chocolate/coffee/beer, you can use a:

Would you like a white chocolate?

as you can say

Would you like a coffee/beer?

When referring to the single piece on the table, you can also use the:

Would you like the white chocolate?

This can refer to the specific piece on the table. However, in other contexts, it can also refer to the substance white chocolate in general:

Do you want the white chocolate or the dark chocolate?

Note that the usage of the indirect article does not work for all mass nouns. We (at least, I) would not say

??Would you like a cake?

It would have to be:

Would you like a piece of cake?