Preheat oven to 350°F (170°C). Grease a 9x13 inch (25x35 cm) baking pan.

For the cake, in a large mixing bowl, whisk together the flour, salt, granulated sugar and baking powder. In a medium bowl, whisk the milk, eggs, vanilla and the ⅓ cup (75 g) melted butter. Pour the milk mixture into the flour mixture and whisk to combine. Pour the cake batter into the pan and set aside.

For the Cinnamon Swirl Filling, whisk the ½ cup (112 g) melted butter, brown sugar, 1 tablespoon flour and the cinnamon until well combined. Spoon or pour the filling in lines over the cake. Swirl the filling into the cake batter with a butter knife, making long swoops lengthwise and crosswise across the pan. Bake for 18-25 minutes, until a toothpick inserted into the cake comes out clean. Cool in pan on a wire rack.

As soon as the cake comes out of the oven, make the Vanilla Icing. Whisk the powdered sugar, vanilla extract and heavy cream until smooth, adding more cream if necessary until icing is pourable. Drizzle icing over warm cake. Serve cake warm or at room temperature. Store covered at room temperature for up to two days or in the refrigerator for up to four days.