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Preheat oven to 350 degrees and grease and dust a baking sheet with butter and flour.
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Place butter, cocoa, and 1 1/2 cups of water in a small saucepan over medium-low heat. Whisk often until butter has melted and the mixture is glossy. Remove from heat and set aside.
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In a large mixing bowl, whisk together flours, sugar, baking soda, salt, and cocoa powder. Make a well in the center and pour in cocoa mixture, milk, eggs, and vanilla. Whisk all wet ingredients until combined and then slowly whisk dry ingredients into wet until a thick batter has formed.
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Transfer batter into prepared baking sheet and bake for 25 minutes or until a toothpick comes out of the center clean. Transfer to a wire rack and let cool completely.
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For the frosting: Using an electric mixer with the paddle attachment, beat butter and jam together until incorporated, about one minute. With the mixer on medium-low, slowly add in powdered sugar until a thick frosting has formed.
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Spread icing over chocolate cake and then dust with rasberry dust (if using).
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Slice and serve.