Cake:
Preheat the oven to 375°F (190°C). Grease or line a 9×9-inch baking dish with parchment paper.
In a mixing bowl, beat the egg. Add sugar, milk, salt, oil, and vanilla, and mix well.
Add the flour and sprinkle the baking powder on top of it. Gently mix the baking powder into the top of the flour, then stir the flour mixture into the wet ingredients until fully combined.
Pour the batter into the prepared baking dish.
Streusel:
In a microwave-safe bowl, melt the butter. Stir in the brown sugar, cinnamon, and flour until well combined. Add the chopped walnuts if using.
Sprinkle the streusel evenly over the cake batter. Use a finger or the handle of a wooden spoon to poke the streusel halfway down into the cake batter 12-18 times randomly.
Bake:
Bake for 25-30 minutes, or until a toothpick inserted into the center of the cake comes out with moist crumbs (but not raw batter). Do not overbake to prevent the cake from drying out.
Let the cake cool slightly.
Icing (Optional):
In a small bowl, mix the butter and powdered sugar. Gradually add milk until the desired consistency is reached.
Drizzle the icing lightly over the warm cake before serving.