There *is* a magic formula! I knew it. I just knew it.For years I've thought bakers have secret knowledge to which the culinary proles (i.e. me) have not been granted access. I am vindicated. Michael Ruhlman says so.
My success with baked goods has been middling at best, due to a tendency to forget ingredients or make inappropriate substitutions leading to a spectacular collapse equalled only by Australia's batting performance in the Ashes yesterday. If you've ever wondered what made a brownie so gooeylicious and a cupcake so undefinably distinct from a muffin, I am come to the rescue, armed with references and of course, a graph.
These three goods belong to the 'batter' category of baking - a pourable mixture with more liquid than solid. The key factors determining the nature of a batter end product are 1) the mixing method and 2) the ratio of the key ingredients (some or all of: flour, fat, eggs, sugar, liquid).
Mixing: Cupcakes (usually based on a sponge cake recipe) are light-footed airy little pixies. They require the sugar and eggs to be beaten first (called foaming), then the butter and flour added afterward. Sidenote: if you use the exact same ratios but start by creaming the butter and sugar, adding eggs and flour after, you get a much denser pound cake as a result.
Muffins and brownies* are both quick breads, where the wet and dry ingredients are mixed separately and then stirred together, just enough to combine them. Quick breads are leavened by baking powder, whereas cakes don't necessarily need it if you are skilled enough to get exactly the right amount of air into the mixture when beating it. But why not take advantage of modern (early Victorian) science and make life easy for yourself?
Ratios: Below are the relative ratios for cupcakes, muffins and brownies. The cupcake is the base case, using equal proportions of flour, fat, eggs and sugar for a light but firm bite. The muffin and brownie both contain additional liquid (milk and chocolate respectively) adding moisture to the final result. Meanwhile, in the brownie sugar has replaced about 1/4 of the flour (vs. cupcake) leading to a weaker structure and slightly gelatinous consistency.
Voila! Secret staircase revealed. You can use these ratios in any quantity to make the kind of tasty treat you love best. But for those interested in *why* these ratios make such a difference, I will turn to my other favourite cook book: McGee on Food and Cooking which discusses the properties of each ingredient.
Flour: When wheat flour is mixed with water, the glutenin protein molecules link up end to end to form long, composite gluten molecules, giving the dough both elasticity (resists pressure and moves back towards original shape) and plasticity (changes shape under pressure). 'Working' dough allows more of these long chains to link up, strengthening the structure -- necessary in yeasted breads, but giving undesirable toughness to cakes and shortbread. This is why cake recipes often warn against over mixing, and flour is usually put in last.
Starch makes up about 70% of wheat flour. Starch granules interpenetrate the gluten network, breaking it up and so tenderising it. In cakes, starch is the major structural material as gluten is too dispersed in the large amount of water and sugar to contribute to solidity. During baking, starch granules absorb water, swell and set to form the rigid walls around the carbon dioxide bubbles, containing them and maintaining the structure.
Fat: Fats and oils 'shorten' a dough or weaken the structure thus making the final product more tender and flaky (e.g. pastry). in rich breads and cakes, fat bonds to parts of the gluten protein coils and prevent the proteins from forming strong gluten.
Sugar: As well as adding sweetness, sugar retains moisture, and limits the development of gluten. Hence the more moisture you like your brownies to have, the more sugar they will need.
Eggs: Are the magical all rounder ingredient, doing a great deal of work for your baked goods. The proteins provide some of the structure that holds the cake together. The yolk contains emulsifiers that help the other ingredients blend together, fats that make it richer and better-tasting as well as softening the texture of the cake, keeping it from becoming chewy.
A note on gluten free baking: Gluten free flours such as rice and sorghum are often supplemented by separate starches e.g. tapioca and xanthum gum. The gum, which is secreted by a bacterium and purified in industrial scale fermenters, provides a gluten like elasticity.
Some time ago the on-line gluten free community started a project called the gluten-free ratio rally to better understand how to transpose recipes into their GF equivalents. You can find more on it and recipes here, here and here:
*At least, the recipe that i've used from my Usborne First Book of Cookery is. Brownie recipes vary wildly.