Preheat the oven to 375°F.

In a mixing bowl, beat the egg. Add the sugar, milk, salt, oil, and vanilla extract, mixing until combined.

Spoon the flour into a measuring cup, then dump it on top of the batter. Sprinkle the baking powder over the flour, then gently mix the baking powder into the top half of the flour. Stir the flour mixture into the batter until just combined. Pour the batter into a greased or parchment-lined 9x9-inch baking dish.

In a microwave-safe bowl, melt the butter. Remove from the microwave and stir in the brown sugar, cinnamon, and flour until well combined. Add chopped nuts if desired.

Sprinkle the streusel evenly over the cake batter. Using a finger or the small end of a wooden spoon, poke the streusel randomly 12 to 18 times halfway down into the batter.

Bake for 25 to 30 minutes, or until a toothpick inserted into the center comes out with moist crumbs (not wet batter). Be careful not to overbake, as this can dry out the cake.

If desired, mix the butter and powdered sugar, then thin with milk until you reach your preferred consistency. Lightly drizzle the icing over the cooled cake.

For the cake batter, spoon the flour into the measuring cup for accurate measurement. For the streusel topping, you can scoop the flour directly with the measuring spoon.

Tip: Avoid overbaking to keep the cake moist and tender.