Servez-vous du gâteau si vous en voulez encore.

Because this is a command: Help yourself.

French forms the affirmative imperative by using the verb without the subject pronoun, and with a pronominal verb like se servir, the pronoun comes after the verb with a hyphen:

• vous vous servez = you help yourself / you are helping yourself
• servez-vous = help yourself / help yourselves

So servez-vous is the imperative form of se servir here.

Here, se servir de quelque chose means to help oneself to something.

So:

• se servir de gâteau would be the basic pattern
• in real French, with an article: se servir du gâteau

This expression is very common when offering food or drink. It has the idea of go ahead and take some for yourself.

Because the verb phrase is se servir de + noun.

With le gâteau, de + le contracts to du:

• de + le gâteau → du gâteau

So servez-vous du gâteau is built from the structure se servir de quelque chose.

A useful nuance: in meaning, it often feels like help yourself to some cake, but grammatically du here comes from de + le after se servir de.

En replaces du gâteau so French does not have to repeat it.

Compare:

• Si vous voulez encore du gâteau = If you want more cake
• Si vous en voulez encore = If you want some more

With verbs like vouloir, French often uses en to refer back to something introduced by de/du/de la/des.

So here en means something like some of it or any more.

Here encore means more / some more / another helping, not really again in the usual English sense.

So:

• vous en voulez encore = you want some more
• in this context, it often suggests seconds

Encore can mean again in other sentences, but here the food context makes more the natural meaning.

Because si vous en voulez encore is not a question. It is a subordinate clause meaning if you want some more.

French uses inversion like voulez-vous ? in direct questions:

• Voulez-vous encore du gâteau ? = Do you want more cake?

But after si, you use normal word order:

• si vous en voulez encore = if you want some more

It can be either:

• formal singular: speaking politely to one person
• plural: speaking to more than one person

So Servez-vous du gâteau can mean either:

• Help yourself to the cake
• Help yourselves to the cake

The form is the same.

It would be:

Sers-toi du gâteau si tu en veux encore.

A couple of things change:

• servez-vous → sers-toi
• vous → tu
• voulez → veux

Notice that in the affirmative imperative, te becomes toi:

• tu te sers → sers-toi

That is a special feature of affirmative imperative forms with me/te.

Yes, but the nuance is slightly different.

• Servez-vous du gâteau = Help yourself to the cake
  This sounds like an invitation to take some for yourself, often from a shared dish.
• Prenez du gâteau = Take some cake
  This is also correct, but a bit more direct and a little less idiomatic in this hospitality context.

So servez-vous is especially natural when offering food politely.

Often, yes.

Because of encore, the sentence can suggest more cake or another piece, so it often sounds like an offer of seconds.

But depending on context, it can also simply mean:

• help yourself to the cake if you still want some

So the exact implication depends on the situation, but encore often points toward more of what was already being eaten or offered.