Mix the first 4 cake ingredients in a bowl. Make a well in the center, and add the wet ingredients. Mix just until all ingredients are incorporated. Pour into a greased 8 or 9 inch cake pan (round or square).
Place brown sugar, flour and cinnamon for streusel into a small bowl. Cut butter into dry ingredients. Add chopped nuts, or more flour if omitting nuts. Crumble topping over batter .
Bake at 350 for about 30-40 minutes, or until toothpick inserted in center comes out clean. Serve warm.