Preheat oven to 375 degrees F (190 degrees C). Grease an 8-inch square baking pan and set aside.
To make the streusel, combine 1/3 cup pancake mix, brown sugar, and cinnamon in a mixing bowl. Cut it the margarine until mixture is crumbly. (This can also be done in the food processor: pulse mixture 2 to 3 times to combine.)
Stir together the 2 cups of pancake mix, soy milk, sugar, and egg just until combined. Spread into prepared pan. Sprinkle with cinnamon streusel.
Bake in preheated oven for 20 to 25 minutes, or until a toothpick inserted into the center of the cake comes out clean. Cool before serving.