Easy Chocolate Sheet Cake

For the cake

• 1 cup water
  
• 1/2 cup (8 tablespoons) unsalted butter
  
• 1 1/2 ounces unsweetened chocolate
  
• 2 cups (8 1/2 ounces) flour
  
• 2 cups (14 ounces) granulated sugar
  
• 2 eggs
  
• 1/2 cup sour cream
  
• 3/4 teaspoon espresso powder (optional, but enhances chocolate flavor)
  
• 1 teaspoon baking soda
  
• 1/2 teaspoon salt
  

For the frosting

• 1 cup (16 tablespoons) unsalted butter
  
• 6 tablespoons milk
  
• 1 1/2 ounces unsweetened chocolate
  
• 4 1/2 cups (About 16 ounces) sifted confectioners' sugar (start with less and add as needed)
  
• 1 teaspoon vanilla
  

1. Preheat the oven to 375 degrees F. Grease and flour a 9" x 13" pan.
2. To make the cake: In a large saucepan, combine the water, butter, and chocolate. Bring to a boil, stirring occasionally, then remove from the heat.
3. Add the flour, sugar, baking soda, salt, and espresso powder (if using) to the saucepan and whisk to combine.
4. Add the eggs and sour cream and mix well. Pour the batter into the prepared pan and bake for 20-25 minutes, until the cake begins to pull away from the sides of the pan and springs back when touched lightly.
5. To make the frosting: Add butter, milk, and chocolate to a saucepan and bring to a boil, stirring to melt the chocolate.
6. Remove from the heat and add the sugar -- you may not need all the sugar so start with half or so and add to get to the right consistency. Beat with a handheld mixer (or transfer to the bowl of a stand mixer) until the frosting is smooth. If it is too liquidy, refrigerate the frosting for a bit and then try beating again. It'll thicken up as it cools and become nicely spreadable. Mix in the vanilla once the consistency is right.
7. Spread the frosting over the cake, then cut into squares and serve.