what does the yolk of the egg do to a cookie recipe or any other, because some recipes ask for dif. amount of eggs and egg yolks?
Eggs are often emulsifiers that also provide a little structure, except in meringues and sponge cakes where they whip into foams that are the main source of structure in a recipe. Yolks emulsify, provide richness and flavor, and tenderize/moisturize, while whites will also emulsify but do not give a rich flavor and tend to dry out a recipe unless balanced with other ingredients (sugar, butter, etc.). A recipe with all yolks will generally be richer, denser and more flavorful, while one with all whites will generally be lighter in both texture and flavor. Whites will whip up into the largest volume foam, which can be useful in some recipes, either where the lightest possible texture is the goal, or when trying to lift heavy ingredients like chocolate.
how if it can be done to marbleize cookies?
Make two separate doughs (or add melted chocolate to half a batch of dough), then roll them separately into flat rectangles. Place one rectangle on top of the other and roll up into a long spiraled roll. Refrigerate until firm, then slice and bake.
what is a good substitute for corn starch?
Depends on the recipe. As others have mentioned, flour, cassava, arrowroot, etc. Be careful with amounts, as these do not all thicken at the same rate. Flour will produce a more grainy texture than cornstarch and cassava will produce a more shiny, stringy texture.