Preheat the oven to 325°F. Spray a 15x10x1-inch jelly roll pan with baking spray. Set aside.
Toast Pecans: On a separate sheet pan spread pecans in a single layer. Toast for 6-8 minutes or until fragrant. Set aside.
Cake Batter: In a medium-size mixing bowl, use a whisk to sift together flour, granulated sugar, brown sugar, baking soda and salt.
In a separate bowl, stir together, boiling water and cocoa until dissolved. Add melted butter mix well.
Pour the cocoa mixture over the flour mixture. To the bowl add buttermilk, vanilla and eggs. Mix with a whisk until fully combined.
Pour cake batter evenly into pan. Bake for 25-30 minutes or until a toothpick inserted into the center comes back clean.
Fudge Frosting: 5 minutes before the end of baking, begin preparing the frosting.
In a medium-size saucepan over medium-high heat melt together butter, cocoa powder and cream. Bring to a simmer, whisking until smooth. Do not boil. Remove from the heat. Add vanilla.
Use an electric mixer to beat in the powdered sugar one cup at a time. Beat until smooth. Mix in toasted pecans. Spread evenly on hot cake. You can add additional cream 1 Tbsp at a time, if needed to thin. Don't thin the icing too much, it will melt on the hot cake making it easy to spread.
Place on a cooling rack to cool in the pan. Store at room temperature or chilled.