Preheat & Prep PanHeat oven to 350°F (177°C) and grease a 9×13-inch baking dish with nonstick spray.
Mix the Cake BatterIn a large bowl or stand mixer, whisk together flour, sugar, baking powder, eggs, vanilla, and milk until smooth. Slowly stream in melted butter, mixing just until combined
Spread Batter in PanPour the batter into the prepared baking dish and smooth it into an even layer.
Make the Cinnamon FillingIn a separate bowl, mix softened butter, brown sugar, flour, and cinnamon until creamy and uniform.
Create the SwirlDrop spoonfuls of cinnamon filling over the cake batter. Use a butter knife to gently swirl it in, creating a marbled effect.
BakeBake for 35–40 minutes, or until a toothpick inserted into the center comes out clean.
Glaze the CakeWhile the cake is still warm, whisk together powdered sugar, milk, and vanilla. Drizzle evenly over the warm cake so it soaks in.
Serve & EnjoyLet the glaze set for a few minutes, then slice and serve warm or at room temperature.