Preheat oven to 350° F then spray 8 x 8 baking dish with non-stick cooking spray.
Crumb Topping:
In a medium bowl, mix together brown sugar, sugar, and cinnamon until incorporated. Pour in melted butter and stir to combine.
Add flour and mix until no streaks of flour are left, then set aside.
Cake:
In the bowl of a stand mixer, or using electric beaters, cream together butter and sugar until fluffy. Mix in egg and vanilla then slowly add milk and mix until incorporated.
Add flour, baking powder, baking soda, and salt and mix until fully combined scraping down sides as needed.
Pour batter into prepared baking dish spreading into even layer. Crumble topping and even distribute on top of batter.
Bake at 350° F for about 30-35 minutes or until toothpick inserted in center comes out clean.