Preheat oven to 350F (180C).

Combine flour, sugar, salt, and baking powder in a bowl.

Melt butter in a saucepan.

Stir in cocoa until dissolved.

Add boiling water and boil for 30 seconds.

Remove from heat and pour over dry ingredients, stirring well.

In a small bowl, mix yogurt, eggs, and vanilla extract.

Pour yogurt mixture over chocolate mixture and stir until combined.

Pour batter into a rectangular baking pan.

Bake for 20 minutes, or until a toothpick comes out clean.

For the frosting, melt butter in a saucepan.

Stir in cocoa until dissolved.

Remove from heat and add milk, vanilla, and powdered sugar.

Pour frosting over the hot cake.

Cut into squares.