Preheat the oven to 350°F (175°C). Line a 12 x 17-inch baking pan with parchment paper.
In a large bowl, mix together the sugar, flour, cocoa powder, baking powder, baking soda, and salt.
Add the sour cream, canola oil, and vanilla extract to the dry ingredients. Mix with a wooden spoon until just combined.
Pour in the boiling water and slowly mix until the batter is smooth and liquidy.
Pour the batter into the prepared pan, spreading it evenly. Bake for 10 to 12 minutes, or until the cake is springy to the touch.
Let the cake cool completely. Use a 2-inch cookie cutter to cut out cake circles and place them on the bottoms of a mini cheesecake pan with removable bottoms.
To prepare the Chocolate Mousse:
Create a double boiler by bringing water to a soft simmer in a saucepan. Place the chocolate, butter, espresso, and sugar in a heat-resistant bowl and allow them to melt over the simmering water.
Let the mixture cool to room temperature.
Whip the heavy cream until it becomes stiff. Reserve one cup of the whipped cream for topping.
Add the remaining whipped cream to the chocolate mixture and fold gently with a spatula until it becomes smooth and fully combined.
Pour the chocolate mousse evenly into the mini cake pan, spreading it gently to level the top.
Refrigerate the cakes for at least two hours.
To prepare the Chocolate Ganache:
Melt the chocolate and heavy cream in a microwave-safe bowl on high, using 20-second intervals. Stir between each interval until the mixture is completely melted and smooth. Let cool slightly.
Gently push the mini cakes up from the bottoms to remove them from the pan. Place the cakes on serving plates.
Pour the ganache over the top of each cake.
Pipe the reserved whipped cream on top, and garnish with chocolate shavings or sprinkles.