Preheat your oven to 350°F.
Grease your 9x13 inch pan well.
Line it with parchment paper.
This prevents sticking later on.
Set the pan aside now.
Mix flour, baking powder, and salt.
Use a separate bowl for this.
Beat butter and sugar until fluffy.
Add eggs one at a time.
Pour in vanilla extract next.
Add flour mixture and milk alternately.
Mix until just combined smooth.
Don't overmix the batter please.
Combine flour, brown sugar, and cinnamon.
Add melted butter to this mixture.
Use a fork to mix.
The mixture should be crumbly.
It should clump when squeezed.
This makes perfect crumbs on top.
Pour batter into prepared pan.
Spread it evenly with spatula.
Sprinkle crumb topping all over.
Cover the entire surface completely.
Press crumbs down gently now.
Place pan in preheated oven.
Bake for 40-45 minutes total.
A toothpick should come out clean.
The crumbs turn golden brown.
Let it cool completely before cutting.