Preheat oven to 350°F. Bring first 3 ingredients to a boil in a medium saucepan over medium-high heat, stirring often. Remove from heat.
Whisk together flour and next 4 ingredients in a large bowl until blended; add warm soft drink mixture. Whisk in buttermilk, eggs, and vanilla. Pour batter into a lightly greased 17 1/2- x 12 1/2-inch jelly-roll pan.
Bake at 350°F for 18 to 22 minutes or until a wooden pick inserted in center comes out clean.
Prepare Fudge Frosting: Heat butter and chocolate in a medium saucepan over medium-low heat, stirring constantly until melted and smooth. Remove from heat and whisk in milk and soft drink until blended. Stir in sugar and vanilla. Beat at medium speed with an electric mixer until smooth and sugar dissolves. Pour over warm cake, spreading gently to edges. Sprinkle with chopped pecans. Cool completely in pan (about 1 hour).