Preheat the oven to 325 F. Sift together the flour, sugar, baking soda, and salt in a large bowl. Stir in Chobani Honey Greek Yogurt, sour cream, and lightly beaten eggs. Melt the butter in a heavy saucepan over medium heat. Whisk in 1 cup water and 1/4 cup cocoa. Bring to a boil, whisking constantly. Remove from heat.
Let it cool a minute, and then with a hand-mixer on a low speed, add in the yogurt mixture into the cocoa mixture a little at a time. Pour the luscious batter into a lightly greased 15x10x1 jelly-roll pan.
Bake at 325 F for 25-30 minutes or until a toothpick inserted in the middle comes out clean.
While the cake bakes, prepare the frosting.
Stir together first 3 ingredients in a medium saucepan over medium heat. Cook, stirring constantly, 2 minutes or until butter is melted. Cook, stirring constantly, 2 minutes or until slightly thickened; remove from heat. Beat in powdered sugar and vanilla at medium-high speed with an electric mixer until smooth. Spread the warm cake with Fudge Icing.