This recipe comes from The Cake Bible by Rose Levy Barnbaum. I use my copy so often that it falls open to all the right pages and the whole thing is covered with little stains and chocolate smears. This book should be in every cook's kitchen. I think the best thing about this book is how Rose goes into why things work and gives you understanding so you can start to make changes on your own.
Raspberry Puree Sauce
2 12oz bags of frozen raspberries with no sugar added
2t lemon juice
Thaw the berries in a strainer over a medium sized glass bowl. I usually do this over night. Once the berries are completely thawed they should have given off about 1 cup of juice. You now are going to microwave the juice on high until the juice is reduced to ¼ cup. In my high powered microwave it takes about 6 minutes. You can do this on the stove as well, but I found that it was easy to burn or over caramelize the juice on the stove.
Puree the raspberries in a food processor and the pass them through a fine sieve to remove all of the seeds. Once you have passed all of the berries through the sieve you should have about 1 Cup of raspberry puree in the bowl. Now combine the concentrated juice with puree and add the lemon juice. At this point I taste the sauce to see how much sugar I want to add. I find that I rarely use all 2/3 Cup of sugar. It just depends on how sweet the raspberries are.