1/2 cup pure heirloom Greek yogurt
1 1/2 tablespoons vegetable oil
1/2 teaspoon pure vanilla extract
1 1/2 teaspoons lemon zest
1 cup brown sugar
1 cup cooked buttermilk instant coffee granules
Make the yogurt and oil: Heat a 10x15 inch dish or large mixing bowl over medium heat. Add enough cold water to thin enough to cover bottom. Dissolve the lemon zest in 1 cup brown sugar and stir into a small bowl; beat into the brown sugar mixture. Spread batter into 12x14 inch loaf pan.
Place the brown sugar mixture on a serving plate. Spread remaining butter on top of the brown sugar mixture. Continue to spread brown sugar mixture on top of remaining butter. Temporarily top off brown sugar with lemon zest. Chill in refrigerator.
Heat the remaining 1 cup brown sugar and lemon zest in a small saucepan, over medium heat. Stir constantly until chocolate has melted and blended with brown sugar. Pour over loaf in pan. Fold over top of loaf. Chill overnight.
Remove loaf from refrigerator. Return loaf to pan with brown sugar layer; sprinkle with remaining butter. Roll out sides of pan to allow. Refrigerate overnight.
The next morning, preheat oven to 375 degrees F (190 degrees C). Roll out bottom crust of loaf; cut away crust to form rosette. Brush chocolate coat onto top crust. Place loaf on ungreased oven rack; bake at 375 degrees F (190 degrees C) for 60 minutes.
Watch the marshmallows rolling up the sides of the loaf. Flip loaf braids. Brush chocolate on top of loaf. Bake at 375 degrees F (190 degrees C) for 15 to 20 minutes.