For the cake:
1 1/2 cups flour
3/4 cup unsweetened cocoa
1 1/2 tsp baking powder
1/2 tsp baking soda
1/2 tsp salt
1 cup sugar
1/2 cup light brown sugar
2 large eggs plus 1 egg yolk, at room temperature
1/2 cup milk, at room temperature
1/2 cup plain fat-free Greek yogurt, at room temperature
1/2 cup vegetable or canola oil
2 tsp vanilla extract
1/2 cup boiling water
4 oz semisweet chocolate, chopped
For the topping:
3/4 cup cherry preserves
For the frosting:
1 cup unsalted butter, softened
1/4 tsp salt
2 tsp vanilla extract
4 cups powdered sugar
2 Tbsp milk
Preheat oven to 350 °F and spray a 13 x 9-inch baking pan with nonstick spray. Whisk together flour, cocoa, baking powder, baking soda, and salt in a large bowl; make a well in the center of the mixture. Add sugars, eggs, egg yolk, milk, yogurt, oil, and vanilla extract to flour mixture; whisk in just until incorporated. Add boiling water to batter and whisk in just until smooth. Fold in chopped chocolate…
Pour batter into the prepared baking pan and smooth with an offset spatula. Tap pan sharply to reduce air bubbles. Bake for 24 to 26 minutes, until a toothpick inserted in the center comes out clean or with a few crumbs, and the edges are pulling slightly away from the pan. Cool cake completely in a pan on a wire rack.
Spread cherry preserves over cooled cake with an offset spatula; transfer to freezer and chill for 20 minutes to set.
For the frosting, beat butter, salt, and vanilla in a large mixer bowl on medium speed until combined. Reduce speed to low; add powered sugar 1 cup at a time and beat well, scraping down bowl as needed. Add milk and beat on medium-high speed until light and airy.
Dollop frosting over cake and gently swirl with an offset spatula. If desired, top with fresh cherries and chocolate shavings. Chill cake for 20 minutes in the refrigerator before slicing and serving.