This is the foundation for a light and fluffy cake. In a large mixing bowl, cream the softened butter and sugar with an electric mixer on medium speed until light and airy. This process takes about 3 minutes, so be patient!

One at a time, beat in the eggs, scraping down the sides of the bowl after each addition. Ensure each egg is fully incorporated before adding the next.

Whisk the flour, baking powder, and salt together in a separate bowl. Gradually add this dry mixture to the wet ingredients, alternating with a splash of milk (not included) if needed to achieve a smooth batter. Don't overmix – a few lumps are okay!

Preheat your oven to 350°F (175°C) and line your 9x13 inch baking pan with parchment paper for easy removal. Pour the batter into the prepared pan and set it aside.

While the cake bakes, prepare the streusel topping, which adds a delightful textural contrast. Use a fork or pastry cutter in a small bowl to combine the softened butter, brown sugar, flour, cinnamon (if using), and salt until crumbly.

Sprinkle the streusel topping generously over the cake batter. Bake for 30 minutes or until a toothpick inserted into the center comes clean. The streusel should be golden brown and fragrant.

Let the cake cool completely in the pan before slicing and serving. The wait might be challenging, but the contrasting textures and rich flavors are worth it!