Preheat the oven to 350℉. Butter and flour a 8x8-inch pan.
In a large bowl, whisk together the flour, sugar, baking powder and salt. Make a well in the middle of the dry mixture. Pour in the milk, butter, egg and vanilla. Stir the mixture together until well-incorporated, then pour into your prepared pan. The batter may seem like not enough, but the cake rises quite a bit during baking.
MAKE THE CINNAMON SWIRL:
In a medium bowl, stir together the butter and brown sugar until smooth. Add in the nuts (if using), flour and cinnamon; stir until smooth. Scoop spoonfuls of the mixture across the top of the cake batter in the pan. Run a butter knife across the cake several times to swirl the mixture.
Bake the cake for 25 to 30 minutes or until the center is set and a toothpick inserted into the cake comes out nearly clean. Let the cake cool slightly while preparing the glaze.
PREPARE THE GLAZE:
In a medium bowl, stir together the sugar, milk, vanilla and salt until smooth. Drizzle the glaze over the cake and serve it warm, while it is still gooey.
Notes
Store this cake (covered) in the fridge for up to one week.