Cake
1-Preheat oven to 375°: Preheat oven to 375°.
2-Beat egg in mixing bowl: Beat egg in mixing bowl. Mix in sugar, milk, salt, oil, and vanilla.
3-Spoon flour into measuring cup: Spoon flour into measuring cup. Dump flour on top of the batter. Sprinkle baking powder over flour and gently mix into the top half of the flour. Stir flour mixture into batter until combined. Pour into greased, or parchment lined, 9×9 baking dish.

Streusel
4-In a microwave-safe bowl, melt butter in: In a microwave-safe bowl, melt butter in microwave. Remove and mix in brown sugar, cinnamon, and flour and stir until well combined. Add nuts if desired.
5-Sprinkle streusel onto the cake: Sprinkle streusel onto the cake. Use a finger or the small end of a wooden spoon to poke the streusel halfway down into the cake batter, randomly 12-18 times.
6-Bake for 25-30 minutes, until a toothpick: Bake for 25-30 minutes, until a toothpick inserted in the middle comes out with moist crumbs (not runny batter). Do not overbake or it will be more dry!

Icing (optional)
7-Mix butter and powdered sugar: Mix butter and powdered sugar. Thin with milk until desired consistency. Lightly drizzle cake with icing.