Learn English – Will you be having cake

ADDENDUM to Tyler James Young's comment and answers by user3169, nicael, and dantiston
(Please don't upvote this: it doesn't address the main question, the use of futurive will.)

These answers tell you that the there BE construction (the ‘existential’ construction) is not ordinarily used for statements of this sort.

The reasons are complicated—the Cambridge Grammar of the English Language, for instance, devotes five and a half pages to ‘pragmatic constraints’ on the existential construction, and CGEL is just a summary!

But a good rule of thumb is that the primary purpose of this construction is to present its complement, what follows the There BE, as new information—it announces the existence or occurrence of something which the speaker presumes the hearers don’t know about. For example:

There’s a match with Liverpool tomorrow.
There will be a match with Liverpool tomorrow.

This assumes that the hearers don’t know about this match.

Note that the speaker uses a match; the indefinite article also suggests new information. But the match and Liverpool’s match use definite determiners; these mark old information, things that the hearers already know about, so they don’t suit the existential construction very well.

However, there are some circumstances where the existential construction does accept definite complements. For instance:

YOU: I don't see any games worth watching on tomorrow.
ME: Well, there’s Liverpool’s match.

In this case the match information is ‘hearer-old’—I know you already know about it, so I use the. But I believe you have overlooked it, or forgotten it, so I can use the existential construction because it is ‘discourse-new’—I am bringing it into the conversation for the first time.

The premise of your question is wrong. This is because there is no future tense in English. We can refer to future time in many ways. One way is to use the simple present, as in:

1 Unless I see the money on my desk by tomorrow 9am, you're a dead man.

The modal will can also be used to refer to future time. This seems to be the construction you're asking about, so I'll bold an example:

2 Unless they will agree to paint the house red, do not hire them.

(It has been stated that "(this usage of will) is not exactly about the future; it's about volition, i.e. "will" (or willingness)." To which, I reply that as uttered by someone, I am not sure we can neatly cleave willingness and referring to future time. I will go with you is expressing one's present willingness to perform the stated action in the future). Even so, I offer another example:

3 Unless they'll go to see Star Wars tomorrow, I'm not going with them.

But so can about ten other constructions, including these common ones:

4 Unless I am seeing the money on my desk in five minutes, you're a dead man.

5 Unless you are going to tell me the real reason, don't bother to open your mouth.

6 I don't want to bother you for a ride to Boston, unless you'll be driving there anyway.

7 Unless you're about to leave, don't bother washing the car windows.

8 We're going to have way too much work to do unless you are to come to your senses and hire some temps.

(Granted the last construction {to be plus to} is rather stilted and part of 'old school' business language.)

Edited to add:
Other examples of present progressive to refer to future time:

9 Unless I'm sitting here and hearing the sound of your feet walking up the stairs to my office in five minutes, don't bother to come.

10 You can't have any unless you're doing your homework in five minutes.

Also note the response to the question Can I have some candy?:

11 No you may not. Not unless you're doing your homework in five minutes.*

Etc.