I have made this and the people at work(I work at a restaurant/lounge) say it is best they have ever tried including brand names.

3 cups water
2 1/2 cups sugar
1 cup espresso*
1 teaspoon vanilla
2 1/2 cups vodka

*if you don't have means of making espresso, you can use 4 cups coffee in place of the water and espresso

Bring water, espresso, and sugar to a boil. Turn heat down to just barely simmer for 3 hours, mixture should be like a syrup. Cool to near room temperature. Add vanilla and vodka. Stir and refrigerate for 1 week before serving. Makes approximately 1 quart. Store in refrigerator.