Preheat the oven to 350 degrees F. Place the flour, granulated sugar, baking soda and salt in a tight-sealing, 10” spring-form cake pan and whisk to combine. Make a well in the centre of the flour mixture and pour in the 1 cup milk, melted butter, lemon juice and vanilla. Mix the wet ingredients into the dry until well combined. Bake for 40 minutes, or until the cake springs back when gently touched in the very centre.
Cool the cake to room temperature; remove the cake pan’s outer ring. Place the softened butter, icing sugar, 2 Tbsp. milk and cocoa in a bowl and beat until well combined and lightened. Spread the icing on the cake. Refrigerate a few minutes to set the icing, and then enjoy.