For the Cake:
Preheat oven to 350 degrees . In a pan, whisk 2 cups water and the sugar over medium-high heat until the sugar dissolves. Remove from the heat, add the chocolate and butter and let stand 10 minutes; whisk until smooth.
In a bowl, whisk the flour, baking powder, baking soda and salt. Whisk the eggs and vanilla into the chocolate mixture. Add the flour mixture; whisk until smooth. Transfer to foil-lined 9-by-13-inch pan and bake for 35 minutes; let cool in the pan on a rack.
Spread with 2 cups Chocolate Frosting (save the remainder for another use). Cut into 12 pieces; sprinkle each piece with a salty snack, such as potato chips, pretzel fish, trail mix or popcorn.
For the Frosting:
In an electric mixer, beat the butter and salt at medium speed until creamy about 1 minute.
Sift in the confectioners' sugar, 1 cup at a time, mixing at low speed to incorporate (the mixture will be crumbly). Beat at medium speed for 1 minute, then increase to high speed and beat until smooth, 3 to 4 minutes.
Beat in melted and cooled chocolate and vanilla until combined, 1 to 2 minutes. Gradually add the cream then beat until fluffy, 2 to 3 minutes.
