Now that we have all our ingredients, let's start making our Costco chocolate cake. Follow these step-by-step instructions to make sure your cake turns out perfect.

Preheat the oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.

In a large mixing bowl, combine the flour, sugar, cocoa powder, baking powder, baking soda, and salt. Mix well.

In a separate mixing bowl, whisk together the buttermilk, vegetable oil, eggs, and vanilla extract.

Add the wet ingredients to the dry ingredients and mix until just combined.

Gradually add the hot water to the batter, mixing until smooth. The batter will be very thin, but that's normal.

Pour the batter into the prepared baking pan and bake for 35-40 minutes, or until a toothpick inserted into the center of the cake comes out clean.

While the cake is baking, prepare the chocolate mousse. In a medium mixing bowl, beat the heavy cream with an electric mixer until stiff peaks form. Melt the chocolate chips in a microwave-safe bowl in 30-second intervals, stirring between each interval, until completely melted. Add the melted chocolate to the whipped cream and beat until well combined. Set aside.

Once the cake is done baking, remove it from the oven and let it cool completely in the pan.

Once the cake is cooled, use a sharp knife to level the top of the cake, if necessary.

Spread the chocolate mousse over the top of the cake, making sure to spread it evenly.

To make the frosting, beat the butter in a large mixing bowl until light and fluffy. Add the powdered sugar, cocoa powder, milk, and vanilla extract, and beat until smooth and creamy.

Spread the frosting over the top of the chocolate mousse layer, making sure to spread it evenly.

Optional: top the cake with chocolate shavings or other decorations.