Preheat your oven to 375ºF (190ºC). Generously grease the sides and bottom of a 6-inch round pan. Add a bit of flour, and tip the pan from side to side until it’s well coated. Tap the pan to remove any excess flour.
Prepare the crumb topping first. Add the flour, sugars, salt, and cinnamon to a deep bowl; whisk to combine. Using a fork, stir in the melted butter, making sure there are no dry bits of flour.
To make the crumb cake batter, add the flour, sugar, salt, and baking powder to a clean deep bowl. Whisk to blend.
Add the milk, egg, melted butter, and vanilla. Stir with a fork just until combined.
Spoon into the prepared cake tin. Evenly sprinkle the crumb topping over the top, pressing it in gently so it sticks.
Bake until golden brown, and a metal skewer inserted in the center comes out clean, about 35 minutes. Remove from the oven. Set the tray on a wire rack, and allow cake to cool at least 30 minutes before serving. Sift some confectioners’ sugar over the top before serving, if desired.