Set oven rack in middle position. Preheat oven to 350. Line 8x8x2 inch pan with foil and coat or spray with veg. oil.
To make streusel: Place brown sugar, flour and cinnamon in a bowl. Work in butter with fork. Add pecans or any nuts after it is combined.I used almonds.
To make cake: Sift together flour, baking powder and salt. Add sugar. Cut butter into dry ingredients. Lightly beat egg in seperate bowl, add milk, and vanilla and combine. Mix into dry ingredients. Batter will be thick. It can be stored overnight in fridge if desired.
Place half of batter into foiled pan. Sprinkle half of streusel onto batter. Add rest of batter to pan and top with remaining streusel. Bake 35-40 min. or until toothpick comes out clean. Cool on rack. Cut when cooled. Store in wax paper to keep moist.