Grease and line two square cake tins, standard size. Preheat the oven to 180C.
In a bowl, combine the flour, salt, sugar, baking powder, milk, eggs, and vanilla. Mix until combined then stir in the melted butter. Pour out the batter evenly between the two tins.
Combine the softened butter, brown sugar, flour, and cinnamon. Mix using the Kitchenaid or electric hand mixer until well combined. You want a fairly soft consistency.
Drop onto the cake batter using teaspoons, then swirl through the cake. Make sure the brown sugar mixture is in small dollops not large lumps.
Place the tin on a baking tray to prevent spills. Bake for about 25 minutes then check it (may take another 5 minutes). The top should be golden and a toothpick come out clean.
Leave to cool for about an hour, then turn out and turn right way up. To make the glaze, combine icing sugar milk and vanilla and whisk until smooth. Drizzle over the warm cake and serve warm or at room temperature.