4 ounces unsweetened baker's chocolate, melted and cooled
1/4 cup unsweetened cocoa powder
1/4 teaspoon salt
4 cups icing sugar, sifted and divided
1/2 cup buttermilk
1 teaspoon vanilla extract
Sheet Cake: Preheat oven to 375°F. Grease and flour 15- x 10-inch baking pan; set aside. In large bowl, whisk together flour, granulated and brown sugars, baking soda, instant coffee and salt until well blended.
Combine 1 cup water, 1/2 cup butter, oil and cocoa powder in saucepan set over medium-high heat; bring to boil, stirring constantly. Pour over flour mixture, stirring just until smooth. Let cool slightly; whisk in eggs, buttermilk and vanilla. Scrape into prepared pan; smooth top.
Whisk together peanut butter, melted butter and icing sugar. Drop heaping tablespoonfuls over batter. Using tip of knife, swirl lightly into batter. Sprinkle evenly with 1 cup nuts.
Bake for 20 to 25 minutes or toothpick inserted in center comes out clean. Let cool slightly; turn out onto large serving platter or cutting board.
Chocolate Buttermilk Icing: Using electric mixer, beat butter until light and fluffy; beat in melted chocolate, cocoa powder and salt. Beat in half of the icing sugar. Stir in half of the buttermilk and vanilla. Repeat with remaining buttermilk and icing sugar; beat until smooth and fluffy.
Spread icing evenly over sheet cake. Sprinkle with remaining nuts.