From Anchor's website, http://www.anchorbrewing.com/beer/anchor_porter:
With deep black color, a thick, creamy head, rich chocolate, toffee and coffee flavors, and full-bodied smoothness, Anchor Porter® is the epitome of a handcrafted dark beer.
A blend of specially roasted pale, caramel, chocolate, and black malts, along with our top-fermenting yeast, creates complexity without bitterness. The brew is hopped at a high rate, and naturally carbonated. The result is dark in the glass, but surprisingly light on the palate.
Blend of 2-Row Pale, Caramel, Black & Chocolate
They say they use a 2-row pale malt blend. Using Marris Otter and a 2row pale, or even all MO will give you a good base. You can even add a small amount of Munich malt for added complexity. To get the toffee character, you're probably looking at a crystal 60 or 75. Then chocolate and black patent (black malt). Adjust your ratio of chocolate and black malt depending on if you want to accentuate the chocolate or the coffee and roast flavors. Probably equal parts of each is a good starting point.
Their site says they use Northern Brewer hops. For this beer style, I would estimate they hop to about 35-40 IBUs.
ABV is 5.6%. For this beer, their OG is probably 1.057 - 1.059 and FG 1.014 - 1.016. Good luck, post what you end up brewing and how it compares.