Preheat oven to 350°F. Lightly butter an 8-inch square baking pan and line bottom with wax paper, then lightly butter paper.
Beat together butter, sugar, and zest in a large bowl with an electric mixer until creamy, about 2 minutes. Add eggs 1 at a time, beating until just incorporated. Whisk in the yogurt and orange juice, then add remaining ingredients, whisking until just combined.
Transfer batter to baking pan and bake in middle of oven until golden and a wooden

Frosting

Beat the butter with a hand mixer until creamy

Add the sieved and mixed confectioners sugar and cocoa in 3 parts while beating with the hand mixer

Add vanilla and salt

Add milk or heavy cream to achieve your desired consistency

Lastly add your melted chocolate and mix well