July 1 and I have coffee.
Given that July is approx. 10 weeks away, we are limited in the target %ABV. You said 8%ABV, which approaches Old Ale territory. Personally, I'd think more like 6.5%ABV since with any value greater extended conditioning because adviseable.
For a 5 gallon batch, all-grain, at a miserable 72.5% efficiency and 72.5% attenuation:
12# Pale Ale Malt, 1# Dark Crystal II (120L), 12oz Chocolate Malt (375L), 8oz Roasted Barley (455L). In this situation, I'd prefer Halcyon pale malt for the base because of it's drier more lager-like character. For the water chem. I'd bias toward the sulfates versus chlorides, holding the mash at 152F.
The Pale Ale Malt is 84.2% of the grist.
The Dark Crystal II is 7% of the grist.
The Chocolate Malt is 5.3% of the grist.
The Roasted Barley is 3.5% of the grist.
With these percentages it should lean in the direction of the coffee/roasted flavour. The colour here should come out around 30SRM and 6.5%ABV. If you want it a touch darker, reserve some of the wort from the mash-tun and semi-carmelize it in an old sauce pan the old lady doesn't mind you ruining.
This formulation could easily become a Coffee-like Chocolate Porter by mashing a few degrees higher, nixing the Roasted Barley and upping the Chocolate Malt and Dark Crystal II percentages to 9% each. It may be even fun to make a version with Baird's darkest crystal (150L).
Maybe someone can chime in on my addition. With some minor exceptions, I'm a dedicated Thomas Fawcett man and seeing as though I haven't used extract in like a decade or so, I may not be the best to give him advice, and perhaps y'all can adjust the formulation for brewing with extract.
But, only having coffee on hand and nothing else. That is good news. There's nothing like a blank slate.