Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan or line it with parchment paper.
In a large mixing bowl, combine 2 cups of flour, granulated sugar, baking powder, and salt. Mix well.
Add the softened butter, egg, vanilla extract, and milk to the dry ingredients. Beat with an electric mixer on medium speed for about 2 minutes until the batter is smooth.
In a separate bowl, prepare the streusel topping by mixing 1/2 cup flour, brown sugar, and cinnamon. Pour in the melted butter and mix until crumbly.
Pour half of the cake batter into the prepared pan. Sprinkle half of the streusel topping over the batter. Pour the remaining batter on top and finish with the rest of the streusel.
Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.