For the cake: Preheat oven to 350 degrees F. Spray a 9x13-inch pan with nonstick cooking spray and set aside.
In a small saucepan, combine the butter, water, cocoa powder, and salt and place over medium heat. Cook, stirring with a spatula, just until melted and combined. Remove from the heat and set aside.
In a large bowl, whisk together the flour, sugar, and baking soda. Add half of the melted butter cocoa mixture and whisk until completely blended. The mixture will be thick. Add the remaining butter mixture and stir until combined.
Add the eggs, one at a time, whisking until completely blended. Add the sour cream and vanilla extract and whisk until smooth.
Pour the cake batter into the prepared pan and bake until a toothpick inserted into the center of the cake comes out clean, 25-30 minutes. Let the cake cool completely before frosting.
For the frosting: In a large bowl, sift together the powdered sugar and cocoa powder. In the bowl of a stand mixer, beat the butter until smooth, about 1 minute. Scrape down the sides of the bowl and add the sifted powdered sugar and cocoa, sour cream, vanilla, and salt. Beat until frosting is smooth, about 3 minutes. If you need to scrape down the sides, do so, and mix again.
Frost the cooled chocolate cake with the chocolate sour cream frosting. Decorate with sprinkles, if desired. Cut cake into squares and serve.