Prepare the streusel by combining the flour, brown sugar, cinnamon, and softened butter, in a mixing bowl. Use a large spoon, silicone spatula, or your hands to crumble the mixture until it looks like wet sand. Set aside.
In the bowl of a stand mixer with the paddle attachment or in a large mixing bowl with a hand mixer, beat the softened butter and sugar until it is pale and fluffy, after a few minutes.
Add the egg and vanilla and beat the mixture again until it’s well combined, after about 1 minute.
Add the milk and mix for 30 seconds.
Add the flour, baking powder, baking soda, cinnamon, and salt. Mix with a large spoon or spatula until you no longer see dry pockets of flour. Scrape the sides and bottom of the bowl to ensure everything is well mixed.
Spread the cake batter into the greased cake pan in an even layer. Sprinkle the streusel topping on top.
Bake for about 30 to 35 minutes, until a toothpick or knife inserted into the center of the cake comes out clean. Slice and enjoy!!
Notes
Softened Butter: Make sure your butter is softened to room temperature. If it's melted or cold and hard, it won't create the right texture in the streusel and cake.
Mix the Streusel by Hand: I find it a lot easier to mix the streusel with my hands rather than with a spoon.
Don't Overmix the Batter: Don't overmix the cake batter, or else it could end up dense. Once you add the dry ingredients, gently mix the batter just until you no longer see pockets of dry flour.
Storing: Store leftover cooled cake wrapped in plastic cling wrap or under a cake dome at room temperature for up to 3 days.