Preheat the oven to 350°F (175°C). Grease and flour an 8-inch round cake pan.
In a medium bowl, sift together the flour, cocoa powder, baking powder, baking soda, and salt. Set aside.
In a large bowl, cream the softened butter and granulated sugar together until light and fluffy, about 3-4 minutes.
Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
Alternately add the dry ingredients and the milk to the butter mixture, beginning and ending with the dry ingredients. Mix until just combined.
Pour the batter into the prepared cake pan and smooth the top with a spatula.
Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and allow it to cool completely in the pan on a wire rack.
Once cooled, cut the cake horizontally into two even layers.
In a shallow dish, combine the cooled espresso and coffee liqueur (if using).
Dip each cake layer briefly into the espresso mixture, making sure not to soak them.
In a large bowl, beat the mascarpone cheese until smooth.
In a separate bowl, whip the heavy cream, powdered sugar, and vanilla extract until stiff peaks form.
Gently fold the whipped cream into the mascarpone cheese until fully combined and smooth.
Place one soaked cake layer in the bottom of a serving dish or trifle bowl.
Spread half of the mascarpone cream mixture evenly over the cake layer.
Place the second soaked cake layer on top and spread the remaining mascarpone cream over it.
Cover and refrigerate the tiramisu for at least 4 hours or overnight to allow flavors to meld.
Before serving, dust the top with unsweetened cocoa powder and sprinkle with grated dark chocolate.