Lightly grease a 9x13 pan with nonstick cooking spray. Preheat oven to 350℉ for metal pans or 325℉ for glass pans. Optionally, line the bottom with parchment paper for easier serving.

In a large bowl, stir together 1 ½ cups all-purpose flour, 1 ½ cups granulated sugar, ¾ cup unsweetened cocoa powder, 1 ½ teaspoons baking soda, ¾ teaspoon baking powder, and ¾ teaspoon salt until combined.

Add 2 large eggs, ¾ cup buttermilk, ¾ cup warm water, ¼ cup vegetable oil, and 1 teaspoon vanilla extract. Use a hand mixer to beat on medium speed until smooth, about 1 to 2 minutes. Use a rubber spatulatto scrape down the sides and bottom of the bowl to make sure everything gets mixed in.

Transfer the batter to the prepared 9x13 pan. Bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean.

Remove the cake from the oven and transfer to a wire rack to cool. Wait to frost until the cake has completely cooled off to room temperature.

Place 1 cup semi-sweet chocolate chips in a microwave-safe bowl. Add ¼ cup corn syrup and 2 tablespoons heavy cream. Heat in the microwave in 30-second increments, stirring between each, until the mixture is fully melted and smooth. Let cool 10 minutes.

Meanwhile, use a large mixing bowl and hand mixer, or use a stand mixer to beat 1 cup salted butter(softened) until creamy, about 2 minutes. Add in ½ cup unsweetened cocoa powder and ½ teaspoon vanilla extract. Beat until combined. Beat in 1 cup powdered sugar, ½ cup at a time, beating in between, until fully combined. Scrape the sides and bottom of the bowl to make sure everything gets mixed in.

Add melted chocolate to the creamed butter. Mix on a medium speed until smooth and fudgy, about 30 to 60 seconds. Spread over completely cooled cake.