ONE-BOWL MILK CHOCOLATE CAKE WITH CHOCOLATE-CARAMEL FROSTING

Steps:

• Bake the cake:
• Preheat oven to 350°F. Butter a 13x9" pan and dust with 2 Tbsp. cocoa powder, tapping out excess.
• Melt remaining 1 cup butter in a small saucepan over medium heat. Add chocolate and remaining 1/2 cup cocoa powder; cook, whisking constantly, until smooth. Let cool slightly.
• Whisk eggs and vanilla in a large bowl. Add sugar and whisk until smooth, lightened in color, and slightly aerated, about 2 minutes. Stream in buttermilk while whisking, then stream in chocolate mixture while whisking until smooth.
• Sift flour, baking powder, baking soda, and salt into chocolate mixture. Whisk until batter is mostly smooth (there will be a few small lumps). Transfer to prepared pan.
• Bake cake until a tester inserted into the center comes out clean, 35-40 minutes. Transfer pan to a wire rack and let cake cool completely.
• Make the frosting:
• Slice 2 oz. chocolate into long, thin shards; reserve for decorating. Coarsely chop remaining 7 oz. chocolate and transfer to a medium bowl. Add vanilla and salt.
• Heat cream in a small pot over medium until just starting to simmer. Pour over chocolate mixture and let sit 3 minutes. Whisk until chocolate is melted and smooth. Whisk in dulce de leche. Chill until firm, at least 1 hour.
• Whisk frosting to lighten and loosen, then use a spatula to spread over cake. If it's too stiff to work with, let it sit out on the counter for a few minutes. Sprinkle with reserved chocolate shards. Cut into thirds lengthwise, then into fifths crosswise (you should have 15 pieces).
• Do Ahead
• Cake can be made 3 days ahead; cover and chill. Bring to room temperature before serving.