The primary issue in substituting between any two dry ingredients in baked goods is the "softness" or fineness to which the material is ground. Cocoa powder is generally about as finely ground as plain flour, so it substitutes most directly in recipes that call for that fine flour. Adding just a little cocoa to a recipe shouldn't require other changes, but more cocoa will need more sugar. If you try to replace as much as half of the flour with cocoa, you will need to add sugar, too, and that may change the moisture and texture of your product.
Substitute 1/4 cup (equal to 4 tbsp) cocoa for 2 tbsp of general purpose flour in a recipe for cookies or something like nut bread. Unless you're sifting the flour, stir the cocoa into the moist ingredients, especially if the recipe starts with creaming butter and sugar together.
Turn an angel-food cake chocolatey by replacing a quarter of the plain flour with the same measure of cocoa powder and sift the two together as directed in the recipe before folding them into the beaten egg white mixture. "Never Fail Angel Cake" in the 1930 original "My Better Homes and Gardens Cook Book" calls for 1 cup of plain flour, sifted and measured, and then sifted four times more. Variation 1 calls for 3/4 cup plain flour and 1/4 cup cocoa.
Use equal measures of cocoa and flour in a sponge cake to fill with something gooey in Swiss roll fashion or a stack of thinner layers. A recipe published by the Wilbur Chocolate Company of Lititz, Pennsylvania, in the early 1900s calls for dissolving cocoa and sugar in water, adding beaten egg yolks, and then sifting in the flour with baking powder and salt before folding in beaten egg whites.
Mix cocoa and flour to your taste, or use cocoa alone, to dust greased layer-cake pans for baking a chocolate butter cake. Mix cocoa with the flour called for to prevent cookie dough from sticking to the pastry board and rolling pin.
Introduce cocoa powder to add a delicate chocolate flavour to your recipe. The richer chocolate cakes and brownies more often use baking chocolate, though cocoa plus shortening may be substituted. Experiment to find the right balance of cocoa, flour and sweetening. Try adding cocoa as a complement to other flavourful ingredients, such as fruits, nuts or shredded zucchini and carrots, and in spice cakes.