With the rack in the middle position, preheat the oven to 350°F (180°C). Line a baking sheet with a silicone mat or parchment paper. Butter the sides of two 8-inch (20 cm) springform pans and line the bottoms with parchment paper.
Place the squash cubes on the baking sheet. Cook for 45 minutes, stirring twice.
In a food processor, purée the squash until smooth. Let cool. You will need 1 ½ cups (375 ml) of the squash purée for the cake.
In a bowl, combine the flour, baking powder, baking soda and spices.
In another bowl, cream the butter and brown sugar with an electric mixer. Add the eggs, one at a time, then the squash purée. Whisk until smooth. Add the flour mixture. Divide the batter between the two cake pans.
Bake for 30 minutes or until a toothpick inserted in the centre of one of the cakes comes out clean. Let cool. Unmould and let cool completely on a wire rack, about 2 hours.
Meanwhile, place the chocolate in a bowl.
In a small pot, bring the cream to a boil. Pour over the chocolate. Let sit for 2 minutes without stirring.
Whisk until the ganache is smooth and creamy. Let cool to room temperature, stirring occasionally, until thick and spreadable, about 3 hours.
Cut each cake in half horizontally to get four layers in total. Divide one-third of the ganache between three of the cake layers. Stack the ganache-covered layers one on top of the other. Top with the remaining cake layer. Completely cover the cake with the remaining ganache.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.