This delicious blackberry cake tasted like the best of late summer in a slice. With sweet-tart blackberry jam between the cake layers, and a creamy blackberry cream cheese frosting, this summery corn cake is the best way to enjoy the fruits of the July, August, and September seasons!
In a medium saucepan, bring all ingredients to a boil over medium heat. Reduce heat to medium low and continue cooking until the mixture thickens, about 8 to 10 minutes, stirring the mixture and crushing the blackberries with the end of a wooden spoon. Remove from heat and set aside to cool completely.
Preheat the oven to 350 degrees Fahrenheit. Mix together the flour, cornmeal, baking powder, baking soda, and salt in a large bowl until well blended. Set aside.
In an electric mixer fitted with a paddle attachment, beat together the eggs, sugar, and honey at medium speed until thick, about 1 minute. Add the olive oil, lemon juice, and vanilla extract and mix until combined. Add the flour mixture to the batter in thirds until completely incorporated, then stir in the rosemary. Evenly distribute the batter between 3 well-greased and lightly-floured 8-inch cake pans.
While the cakes are baking, make the frosting. Beat the butter in the bowl of an electric stand mixer fitted with the paddle attachment at medium high speed until fluffy, about 3 minutes. Add the cream cheese and beat until smooth and fluffy and fully incorporated into the butter mixture, stopping to scrape down the sides of the bowl if needed, beating about 2 minutes more.
Reduce speed to low and add the powdered sugar in 4 increments, mixing until incorporated before adding the next bit. Add the vanilla extract, salt, and 2 tablespoons of the blackberry jam and mix until completely smooth.
Measure out two separate 1/3 cups of the icing and place each 1/3 cup in a separate bowl. Add 1 teaspoon of the blackberry jam to one bowl and stir until smooth. And add 1 *tablespoon* of blackberry jam to the *other* bowl and stir until smooth. So you'll now have three shades of frosting, the palest shade in the big mixing bowl, a medium shade in a little bowl, and the deepest shade in the other little bowl.
Layer the cake with the jam between each layer. Frost the entire outside of the cake lightly with the palest shade (from your large mixing bowl). Ice the bottom third of the sides of the cake with the deepest shade, then ice the middle third of the sides of the cake with the medium shade, and lastly, ice the top third of the sides of the cake with the lightest shade. Serve immediately.
Blackberry Buttercream recipe adapted from America's Test Kitchen.