Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking pan.
Sift together flour, baking powder, 1 teaspoon cinnamon, and salt in a bowl. Set aside.
Combine 2 cups sugar and shortening in a bowl; beat with an electric mixer until fluffy. Add eggs and vanilla; mix well to combine. Pour in flour mixture and milk in batches, alternating between the two and mixing well after each addition. Pour batter into the prepared baking pan and spread out batter towards the edges.
Mix brown sugar, 3 tablespoons white sugar, and 2 teaspoons cinnamon together in a small bowl. Sprinkle topping evenly over the cake.
Bake in the preheated oven for 30 minutes; remove and pour melted butter evenly over the cake. Return to oven and bake until a toothpick inserted into the center comes out clean, about 5 minutes more.
