Preheat oven to 350F. Lightly grease a 7.5 cm round cake pan and line it with parchment paper.
In a bowl, mix together the flour, sugar and salt.
In a saucepan, melt butter and add in the cocoa powder. Stir together. Pour boiling water and allow mixture to boil for 30 seconds. Turn off heat and and pour this over the flour mixture. Stir lightly to cool.
In another bowl add in th butter milk, egg, soda and vanilla. Stir this mixture into butter/chocolate mixture.
Pour into prepare pan and bake for 20 minutes.
Take out the cake and allow it too cool.