I have made it these ways:
HME + 8 oz Golden Light DME
HME + 1x Robust + can blackberries - did not like this.
HME + Pale LME and 2 TBS vanilla extract, was OK once vanilla smoothed out - not a big fan.
HME + 2 Robust + 2 cups oatmeal steeped in bag - Douible Deluxe Oatmeal Porter - very nice
HME + Robust + Smooth + 4 TBS Maltodextrin - good brew.
HME, 1x Robust, 1xSmooth LMEs, 1/4 cup lactose, 1/2 oz EKG 5 min boil, 20 min steep 160 deg 1 oz choc and 1 oz pale choc grains. - Millk Choc Porter - very nice
Also look at the Baltic Porter - it is now a good buy if on sale @ $21. It comes out well. Same cost as Porter with only one Robust. but with strength of HME + 3 Robust. Plus you get a fancy yeast. Also it will be a little more bitter than if you make the 1 regular + 2 or 3 malts. So you save on hops too. If you want it sweeter, add 2-4 oz lactose - cheap enough.