Preheat your Oven and Prepare Your 9×13-inch Cake Pan - Preheat your oven to 350 degrees Fahrenheit. Add a layer of parchment paper to your 9×13-inch cake pan and then spray it with cooking spray.

Beat the Butter and Granulated Sugar Together - Using an electric mixer with a large bowl, beat the butter and granulated sugar together with a whisk attachment. Beat them on medium-high for around two minutes, until it reaches a creamy consistency.

Add the Eggs - Turn the mixer to low speed. Add the eggs, two at a time, until fully combined. Use a rubber spatula to scrape down the sides as needed.

Mix the Dry Ingredients Together - In a medium bowl, use a whisk to mix together the cake flour, baking soda, and salt.

Add the Dry Ingredients to the Wet Ingredients - Add one-third of your dry ingredients into the butter mixture, mixing at low speed. Then, add in half of your buttermilk, continuing to mix at low speed. Next, add one-third of the dry ingredients followed by the rest of your buttermilk and then the rest of the flour mixture.

Add in your vanilla extract and also vegetable oil. Mix the batter until just combined.

Add Batter to Cake Pan and Bake - Add the butter to the cake pan and bake for 25-26 minutes, or until a toothpick inserted into the middle comes out clean. Allow the cake to cool in the pan for 15 minutes before placing it on a wire rack.

Melt the Chocolate Frosting for Yellow Cake - Using a microwave-safe bowl, melt the dark chocolate. Microwave in 30-second intervals, until the chocolate, is smooth.

Make the Frosting - Use a stand mixer with a paddle attachment to beat the butter for about 30 seconds at medium speed. Once the butter is smooth and creamy, turn the mixer to low and add the cocoa powder. Mix together until smooth.

Then, slowly add the powdered sugar to the mixer, just one cup at a time. Alternate with adding small splashes of heavy cream in. Next, add your melted dark chocolate and salt to the frosting.

Beat until all the ingredients have fully combined and the frosting has achieved your desired consistency. If it is too thick, add more heavy cream, and if it is too thin you can simply add more powdered sugar.

Frost Your Cake - Once the cake is completely cool, use an offset spatula to frost the dark chocolate buttercream on the cake. If desired, add sprinkles. Slice your cake and serve it.