Preheat the oven to 375°F. Grease and flour (or using baking spray) 13"x9" baking pan.
Whisk together 3 cups flour, baking powder, cinnamon, salt, and sugar. Cut in the chilled butter until the mixture is coarse and texture is similar to cornmeal.
Beat the eggs. Then add milk and vanilla and whisk together. Pour this wet mixture into the dry flour mixture. Stir to combine, so no dry flour shows, but don't overmix.
Spoon batter into prepared 13"x9" baking pan and spread evenly.
Prepare topping ingredients. Mix the brown sugar, 1/4 cup flour, and cinnamon together making sure to remove any brown sugar lumps. Add the 4T melted butter to this mixture and combine.
Sprinkle the cinnamon streusel topping over the batter and then bake at 375°F for 27-32 minutes or until a toothpick inserted in the center comes out clean.
Let this cool completely and before serving.