Instructions

• Preheat the oven to 350°F. Grease a 9"x13" cake pan (or line it with parchment paper or aluminum foil and grease the sides and bottom). Set the pan aside.
• In a large mixing bowl, whisk together 3 cups sifted all-purpose flour, 1/4 teaspoon salt, 1 cup granulated sugar, and 4 teaspoons baking powder.
• Add 1 and 1/2 cups milk, 2 large eggs, and 2 teaspoons vanilla extract to the dry ingredients and whisk until smooth.
• Slowly whisk in 1/2 cup melted unsalted butter until the batter is uniform.
• Spread the batter evenly into the prepared pan. Set aside.
• In a medium mixing bowl, beat 1 cup softened unsalted butter with an electric mixer until creamy. Add 1 cup packed light brown sugar, 2 tablespoons all-purpose flour, and 1 tablespoon ground cinnamon. Beat about 2 minutes until fluffy — the mixture may look crumbly at first but will come together.
• Spread the filling mixture over the batter, leaving at least a 1-inch border from the edges of the pan. Gently swirl the filling into the batter with an offset spatula or rubber spatula.
• Bake for 28 to 32 minutes, or until a toothpick inserted into the cake portion (not the filling) comes out clean.
• Remove the pan from the oven and cool the cake in the pan on a wire rack for 15 minutes.
• While the cake cools, sift 2 cups powdered sugar into a medium bowl. Add 5 tablespoons milk and 1 teaspoon vanilla extract, then whisk until smooth and of a drizzling consistency (if it seems too thick, add a few drops of milk and whisk again).
• Pour the glaze evenly over the still-warm cake and spread to cover.
• Allow the glaze to set, then store the cake covered at room temperature for up to 3 days.