What are the key requirements, in terms of both ingredients and techniques, for baking chocolate brownies with a fudgy centre?
Every time I make brownies, I always get compliments on how fudgy they are. There may be some variations based on the recipe you use, but the three things I've found to be most important in my experience are:
There's another element to changing the consistency, too: cooking time. Most brownie recipes I've seen say to bake for 35-45 minutes; I almost never bake them for more than 30 because I find that the edges start to get really dried out. Undercooking slightly will keep the brownies moist and fudgy.
The ingredients from the old "Grandmother's Brownies" King Arthur Flour recipe: 4 ounces (squares) unsweetened chocolate [can substitute 1-ounce square = 3 tbs cocoa powder + 1 tbs butter); ½ cup (1 stick) butter; 2 cups sugar and/or brown sugar; 2 to 4 eggs; ½ teaspoon salt; 2 teaspoons vanilla; 1 cup flour
Shirley O. Corriher's Bakewise discusses fudgy vs. cakey brownies. The key is the proportion of fat and chocolate to flour.
Fudgy brownies will have less flour than normal; this will give the brownie less structure via gluten, and gooey middle. Cakey brownies will have more flour.
Shirley's fudgy brownie recipe can be found here: http://www.projectfoodie.com/spotlights/cookbooks/shirley-o.-corriher---bakewise-the-hows-and-whys-of-successful-baking.html. She has thoughtfully measurements in ounces and grams in addition to volumetric measurements.