Preheat the oven to 375 degrees F (190 degrees C). Grease a 9x13 inch baking pan.
In a medium bowl, stir together the flour, sugar, salt and baking powder. Separate the mixture into two bowls - one with about 5 cups, and the other with about 3. In the bowl with the smaller amount, stir in the eggs, oil, milk, vanilla extract and maple extract. Pour this mixture into the prepared baking pan. To the remaining bowl, stir in the butter, cinnamon and walnuts until crumbly and well blended. Sprinkle this over the top of the batter in the pan.
Bake for 35 to 40 minutes in the preheated oven, or until a knife inserted into the center of the cake comes out clean. Dust with confectioners' sugar before serving.