Prepare the cake:
Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans or line them with parchment paper.
In a large bowl, mix together the chocolate cake mix, water, vegetable oil, eggs, and vanilla extract until smooth.
Divide the batter evenly between the two prepared cake pans and smooth the tops.
Bake for 25-30 minutes, or until a toothpick inserted into the center of the cakes comes out clean. Let the cakes cool in the pans for 10 minutes before transferring them to a wire rack to cool completely.

Make the cream filling:
While the cakes are cooling, prepare the cream filling. In a medium bowl, beat together the softened butter, powdered sugar, heavy cream, vanilla extract, and a pinch of salt until smooth and creamy. Set aside.

Prepare the ganache:
In a small saucepan, heat the heavy cream over medium heat until it begins to simmer (do not boil).
Remove from heat and add the semisweet chocolate chips and butter. Stir until smooth and the chocolate is completely melted. Let it cool for a few minutes to thicken.

Assemble the cake:
Once the cakes are completely cool, place one cake layer on a serving plate. Spread the cream filling evenly over the top, leaving a small border around the edges.
Place the second cake layer on top, gently pressing down.

Top with ganache:
Pour the ganache over the top of the cake, allowing it to drizzle down the sides. Use a spatula to spread the ganache evenly over the top if needed. Let the ganache set for 15-20 minutes before slicing and serving.

Serve:
Slice the cake into pieces and serve. Enjoy the homemade version of this nostalgic treat!