Spice Mix
1 Tbsp Cinnamon
1/3 Tbsp Ginger
1/3 Tbsp Nutmeg
1/4 tsp Cloves
For five gallons:
MALT
8# Marris Otter
1# Biscuit malt
8 oz Munich malt
HOPS
1 oz Fuggles (4.0% AA) @ 60 minutes
OTHER STUFF
3# pie pumpkin
4 oz molasses (preferably blackstrap)
4 oz maple syrup (preferably Grade B)
1-3 teaspoons spice mix
Your favorite clean-fermenting yeast (I use US-05 or 1056 these days, but have had good results with WLP002 and WLP007 too).
1.057 OG
1.014 FG
14 SRM
13 IBUs
I prepare my pumpkin for the boil by cubing it (again, roughly in one inch sections) and baking it until it is well browned. Usually this takes about an hour at 375° F, tossing the pan every fifteen minutes or so for even caramelization. After it's browned, I toss it with a mix of brown sugar, a little bit of water, and some spices, and put it back into the oven for about ten minutes. It's almost like you're making caramel corn. After that, I toss the pumpkin into the kettle at the start of the boil and let it go for the full hour. Some people insist this is a bad idea because it will give you cloudy beer -- and you should only use pumpkin in the mash to get the starches converted -- but I've never found this to be true.
The spices. This is incredibly important. Most of the changes I made while I was still developing my recipe involved getting the balance of spices right, both in the actual spice blend and in how much to add to the beer. I like a spicy pumpkin ale; I add a tablespoon of my spice mix at burnout. This is more than most people use, and if you've never brewed a pumpkin ale before you want to start with less. You can always brew a spice tea and add it when you bottle or keg if you find you want more spice character.
Secondary. People do it all the time with fruits, but they never think to add pumpkin in secondary. I've only heard of a handful of other people doing this, but if you really want to see the benefits of using real pumpkin instead of just pumpkin pie spices, you need to do this. Two to three pounds of finely cubed (~0.5") pumpkin, caramelized as before in the oven, then candied with some molasses and maple syrup in a pan on the stove. Leave it for about a week, then straight to the bottle or keg.
Edit: One more thing. Keep your IBUs down. I've seen pumpkin ale recipes stretching into the 30-40 IBUs range, which is way too high in my experience. My recipe sits at 13 IBUs, and I've found that when you go much over 15 or so the hops start to clash with the spices.
Mash at 156° F for one hour, sparge per your setup to collect ~6.5 gallons wort. During your mash your pumpkin should be in the oven roasting, as I talk about in the fourth section of this comment. When I say to toss your cubed pumpkin in brown sugar and spices, I mean about two tablespoons of brown sugar and a very small dash of your spice blend. This is an entirely optional step which probably doesn't make a huge difference; I do it mostly out of habit and tradition. Plus, it smells great.
The rest pretty much takes care of itself, right? Toss your hops, boil for an hour, and kill it. I toss my spices in at flameout. Some people prefer to toss their spices at ~5 minutes to get rid of any 'raw' flavor to the spices, but I find the residual heat between flameout and pitching takes care of that. My spice blend is a mix of one tablespoon of cinnamon, one third tablespoon each of ginger and nutmeg, and a quarter teaspoon of cloves. Use whatever you like. I use a full tablespoon in my beer, but I recommend starting with significantly less if you don't already know how spicy you like your pumpkin ale. If you need to you can adjust the spice character when you bottle by brewing a spice tea and adding it to taste.
The weird part. As I talk about in the sixth section of this comment, I secondary my pumpkin ale for one week with a fair amount (I do 2.5 pounds) of roasted, caramelized pumpkin. I feel this is the step that really puts the 'true' pumpkin flavor into my beer; along with discovering biscuit malt in 1995, I credit it as turning a good pumpkin ale into a great one. This is also where you use the molasses and maple syrup mentioned in the ingredients; toss them into a hot pan with your roasted pumpkin to candy. You can skip the secondary (or split off a gallon or two to try it on a smaller scale) if you find it unconvincing.
