1. Preheat the oven to the required temperature and prepare a bundt pan or cake pans by greasing them.
2. In a mixing bowl, combine the dry ingredients, including flour, cocoa powder, sugar, baking soda, baking powder, and espresso powder.
3. In another bowl, whisk together the wet ingredients such as eggs, mayonnaise, Guinness beer, and vanilla extract.
4. Gradually mix the wet and dry ingredients, stirring until a smooth batter forms.
5. Fold in mini chocolate chips for extra chocolatey texture.
6. Pour the batter into the prepared cake pans and bake until a toothpick inserted in the center comes out clean.
7. Allow the cake layers to cool completely before assembling.
8. Prepare the chocolate pudding or mousse filling by mixing chocolate pudding mix with milk or melted chocolate with whipped cream.
9. If making chocolate ganache, heat heavy cream and pour it over chopped chocolate, stirring until smooth.
10. Once the cake layers have cooled, place one layer on a serving plate and spread a layer of chocolate pudding or mousse filling.
11. Repeat the process with remaining cake layers, ensuring an even distribution of filling.
12. Frost the entire cake with chocolate buttercream, whipped cream, or a smooth chocolate ganache.
13. Drizzle ganache over the top of the cake for a glossy, indulgent finish.
14. Sprinkle mini chocolate chips, toffee pieces, or crushed candy bars on top for added texture.
15. Refrigerate the cake for several hours to allow the flavors to meld and the frosting to set.
16. Slice and serve the cake chilled or at room temperature for a rich, chocolatey indulgence.