Creating the Pumpkin Chocolate Mousse Cake is a journey that is both enjoyable and rewarding. Follow these steps to achieve succulence in every slice:

1. Preheat the Oven: Start by preheating your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.

2. Mix Dry Ingredients: In a large bowl, sift together the flour, granulated sugar, light brown sugar, cocoa powder, baking powder, baking soda, salt, cinnamon, and nutmeg.

3. Combine Wet Ingredients: In another bowl, whisk together the vegetable oil, buttermilk, eggs, vanilla extract, and pumpkin puree until well incorporated.

4. Combine Mixtures: Gradually add the wet mixture to the dry ingredients, stirring until just combined. Do not overmix; a few lumps are okay.

5. Pour Batter: Divide the batter evenly between the prepared cake pans.

6. Bake: Place the pans in the oven and bake for 30-35 minutes or until a toothpick inserted in the center comes out clean.

7. Cool the Cakes: Once baked, let the cakes cool in their pans for about 10 minutes before transferring them to wire racks to cool completely.

8. Make the Chocolate Mousse: In a microwave-safe bowl, combine the chopped semi-sweet chocolate and butter. Heat in 30-second increments until melted and smooth, stirring between each session.

9. Beat Egg Yolks: In a medium bowl, whisk the egg yolks and 1/4 cup of sugar until pale and fluffy. Slowly pour the melted chocolate mixture into the yolks, mixing until incorporated.

10. Whip Egg Whites: In a separate bowl, beat the egg whites until stiff peaks form. Gently fold the egg whites into the chocolate mixture.

11. Whip the Cream: In another bowl, whip the heavy cream and vanilla extract until soft peaks appear.

12. Combine Mousse: Gently fold the whipped cream into the chocolate mixture until well blended and silky.

13. Layer the Cake: Once the cakes are completely cooled, place one layer on a serving platter. Spread half of the chocolate mousse on top.

14. Add Second Layer: Place the second cake layer on top of the mousse and spread the remaining mousse over the top of the cake.

15. Chill the Cake: Refrigerate the cake for at least 1 hour to allow the mousse to set properly.