Preheat the Oven: Begin by preheating your oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan, ensuring it's ready for your delicious creation.
Prepare the Cake Batter: In a large mixing bowl, cream together softened butter and granulated sugar until light and fluffy. Add in the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract.
Combine Dry Ingredients: In a separate bowl, whisk together the all-purpose flour, baking powder, and salt.
Mix Wet and Dry Ingredients: Gradually add the dry ingredients to the wet ingredients, alternating with the milk, until well combined. Mix until just combined; do not overmix.
Make the Crumb Topping: In another bowl, combine granulated sugar, brown sugar, flour, melted butter, and cinnamon (if using). Mix until the mixture resembles coarse crumbs.
Assemble and Bake: Pour the cake batter into the prepared baking pan, spreading it out evenly. Sprinkle the crumb topping evenly over the cake batter, covering it completely.
Bake to Perfection: Place the baking pan in the preheated oven and bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean and the crumbs are lightly golden brown.