Preheat over to 350.
Grease 8x8 pan or line with parchment and spray with cooking spray.
In small saucepan, melt butter, add water ( or milk ) and warm thru.
Add cocoa and baking soda and stir till smooth.
In mixer, whisk buttermilk, egg and vanilla.
Add flour, sugar and salt. Mix until blended.
Spread batter in prepared pan.
Bake 35-45 minutes until tester or toothpick comes out clean.

For frosting:
In saucepan, add butter, buttermilk and cocoa. Heat over medium stirring constantly until smooth.
Remove from heat, stir in vanilla and sugar. Pour over cake while still warm.