Chocolate Cake 

1. Preheat oven to 350F(175C). Grease and line a 9×9 inch square baking pan with parchment paper.
2. Using a fine mesh sieve, sift the flour, cocoa powder, baking powder, baking soda, and salt into a large mixing bowl. Alternatively, the flour, cocoa powder, baking powder, baking soda and salt can be added to a large mixing bowl and whisked to combine.
3. In the bowl of a stand mixer fitted with paddle attachment or with a hand-held electric mixer and large mixing bowl, add the oil and sugar. Mix ingredients at medium speed to combine, about 30-60 seconds. Next, mix in the yogurt into oil-sugar mixture.
4. Beat the eggs into wet batter one at a time allowing the initial egg to be fully incorporate before mixing in the next egg. Next, mix in the vanilla extract into wet batter.
5. Reduce mixer speed to low. In an alternating fashion, starting and ending with the flour mixture, mix half of the flour mixture into wet batter, followed by all of the espresso/strong brewed coffee and ending with the remaining flour mixture.
6. Stop mixer and use a rubber spatula to scrape the sides and base of mixing bowl. At medium speed, mix chocolate cake batter for an additional 30-60 seconds to ensure you have a smooth cake batter.
7. Transfer chocolate cake batter into prepared baking pan and bake for 30-35 minutes or until a toothpick inserted into the center of the cake comes out clean.
8. Allow chocolate cake to cool in baking pan for 5-10 minutes before transferring to a cooling rack to cool completely to room temperature.

Whipped Chocolate Espresso Buttercream

1. In the bowl of a stand mixer fitted with paddle attachment or with a hand-held electric mixer and large mixing bowl, cream the butter and salt at medium high speed until smooth and fluffy in appearance, about 2-3 minutes.
2. Reduce mixer speed to low and mix in 1 1/2 cups of the confectioners’ sugar, cocoa powder, and instant espresso powder (I used 5 teaspoons) into creamed butter. Gradually increase mixer speed back to medium.
3. Once all the dry ingredients have been incorporated into creamed butter, mix in the vanilla extract and 1 1/2 tablespoons of heavy whipping cream into chocolate espresso buttercream until fully incorporated.
4. If buttercream appears to be on the soft side, mix in the remaining confectioners’ sugar 1/4 cup at a time until a thick but spreadable consistency is reached.
5. If buttercream appears too thick, mix in the remaining heavy whipping cream, 1/2 teaspoon at a time until the desired consistency (thick but spreadable) is reached.
6. Increase mixer speed to medium high and mix chocolate espresso buttercream for an additional 30-60 seconds. The resulting buttercream should be smooth and fluffy in appearance.

Assembly

1. Spoon chocolate espresso buttercream onto cooled chocolate cake.
2. Using an offset spatula, evenly spread chocolate espresso buttercream into an even layer on chocolate cake.
3. Sprinkle top of buttercream with chocolate curls, sprinkles, or sea salt (optional).
4. Cut cake into equal pieces and serve.