This moist and rich rocky road cake boasts a thick layer of chocolate sour cream frosting with "rocky road" mix-ins--homemade mini marshmallows and chopped nuts. This is a great cake for a picnic, a pot luck, or to serve for dessert on any day. Easy dessert recipe to make, and even easier to eat! Find the original recipe for chocolate mayonnaise cake with melted chocolate bar frosting in Jill O'Connor's wonderful (affiliate) Cake, I Love You.
Position your oven rack in the center of the oven. Preheat to 350F. Spray a 9"x13" rectangular cake pan with pan spray. Set aside.
Sift together the flour, cocoa powder, baking powder, baking soda, and salt. Set aside.
Stir the espresso powder into the boiling water.
In the bowl of your stand mixer fitted with the paddle attachment, beat the sugar and eggs on medium high speed until they reach the ribbon stage, about 5 minutes.
Add the mayonnaise and vanilla and beat until smooth, scraping the bowl as necessary.
With the mixer on low speed, alternate adding the sifted dry ingredients in three additions with the coffee in two additions. Begin and end with the dry ingredients. Scrape bowl as necessary.
Scrape the batter into the prepared pan and bake for 40-45 minutes or until the cake springs back when pressed in the center, the edges are just starting to shrink from the sides of the pan, and the internal temperature of the cake is between 190F and 200F.
Remove to a rack to cool completely.
To Make the Rocky Road Frosting
Measure out the sour cream and allow to warm up from refrigerator temperature to cool room temperature, about an hour.
Put the semisweet chips, milk chocolate chips, and butter in a microwave safe bowl. Microwave on full power for 1 minute. Stir the chocolate and butter until completely smooth. If it's not all melted evenly, heat in 15 second bursts, stirring in between, until completely smooth.
Whisk in the sour cream until completely incorporated and smooth. Whisk in the salt and vanilla.
Put the confectioners' sugar in a fine mesh strainer and sift it onto the chocolate mixture. Whisk until smooth and shiny.
Fold in about 2/3 of the chopped nuts and mini marshmallows.
To Frost and Finish the Cake
With an offset spatula, evenly spread about half the frosting onto the cooled cake.
Scatter the remaining toasted nuts and mini marshmallows evenly over the frosting.
Dollop on the rest of the frosting. Then use your spatula to swirl and spread the frosting over the nuts and mallows, leaving some of them showing.
Refrigerate the cake until serving time, at least 2 hours. Slice the cake and let slices come to room temperature before serving. Store leftover cake, covered, in the fridge for up to 3 days.
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Notes
To make this a "plain" chocolate sheet cake, leave out the marshmallows and nuts and beat the frosting with a hand mixer or a stand mixer to lighten the texture.
Storing:
Store the cake, covered at room temperature, for 3 days. You can also store it in the fridge. Just allow the cake slices to come to room temperature before serving. Freeze slices for longer storage.
Freezing:
Freeze individual slices, or the whole cake if you'd like. Freeze slices until firm, and then wrap in plastic wrap and foil. To freeze the whole cake, it will be easiest to freeze it in the pan you baked it in. Wrap well with plastic wrap and foil. You can freeze it frosted or unfrosted. If you freeze it unfrosted, bring the cake back to room temperature before frosting.