Prepare the brownie mix according to package instructions except use melted coconut oil instead of melted butter. Mix together by hand in a large bowl until completely incorporated and smooth, but don't over mix. Pour batter into a greased 13x9 baking pan. Bake for 20-25 minutes or until a toothpick inserted in the middle comes out clean. Let cool completely.
Melt chocolate in a glass or metal bowl set over a pot of simmering hot water. Gradually blend in yogurt and it's ready to spread. Frost brownies, then sprinkle with walnuts. Serve immediately or store covered in the refrigerator for up to a week.