It’s the type of cake you’ll keep coming back to, cutting off squares here and there, until you’ve eaten half the cake yourself over a couple days. Spring for the good chocolate and cocoa powder to make it special, but the supermarket standards will also give you great results.
Heat the oven to 350 degrees. Grease a 9-by-13-by-2½-inch metal baking pan with baking spray.
In a small saucepan, melt the butter and chocolate with 1 cup water over medium heat, stirring occasionally. Remove the pan from the heat and let cool to room temperature.
In a large bowl, whisk together the flour, baking soda and salt. In a medium bowl, whisk together the sugar, sour cream, vanilla and eggs until smooth. Pour the chocolate mixture into the sour cream mixture and stir until smooth. Pour the liquid ingredients over the dry ingredients and stir until just combined. Pour the batter into the prepared pan and smooth the top. Bake until a toothpick inserted in the center of the cake comes out clean, 40 to 45 minutes.
While the cake is in the oven, make the frosting: In a large bowl, whisk together the powdered sugar, cocoa powder and salt. Add the butter and beat with a hand mixer on low speed until evenly combined and clumpy. With the mixer still running, slowly drizzle in the cream and vanilla and beat, starting on low speed and gradually increasing to medium-high, until fluffy and spreadable, 1 to 2 minutes.
Remove the cake from the oven, transfer to a wire rack, and let cool completely. Scrape the frosting onto the cake and use a small offset spatula or table knife to spread it evenly over the top before serving from the pan.
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