Preheat oven to 350°F. Spray a 9x13-inch pan with nonstick cooking spray.

Whisk flour, baking powder, and salt in a small bowl. Set aside.

Mix melted butter and sugar in a large bowl with a hand mixer at medium speed. Mixture will be grainy. Mix in egg and vanilla. Stir in milk, then stir in flour mixture until just mixed. Pour cake batter into prepared pan. Set aside.

Cream butter and brown sugar in a large bowl. Mix in flour and cinnamon. Mixture will be smooth. Drop by spoonfuls over cake in pan. Use a butter knife to swirtl the topping throughout the cake.

Bake for 30-35 minutes, or until a toothpick in the cake part comes out clean and it no longer rolls when the pan is tapped. Cool 10 minutes then glaze.

To make the glaze, whisk powdered sugar with vanilla and 4 tablespoons milk, adding more as needed for a drizzle consistency. Drizzle over the top of the cake.

Cool completely or to at least warm (not hot) before serving.

Store covered at room temperature for up to 3 days. Freeze for up to 3 months.