Melted butter is an acceptable substitute for vegetable oil when preparing baked goods, such as cookies and brownies. The butter tends to make the baked item taste richer, but the texture is not affected. The cooking time remains the same when melted butter is substituted for vegetable oil.
Melted margarine also is an appropriate substitute for vegetable oil, although additional cooking time is required. Be aware that vegetable oil spreads are not always a substitute because they are lower in fat, resulting in a drier product. Use a spread that has at least 65 percent fat to keep the baked item moist.