In a small bowl, mix the coffee and Kahlua (if using) to create a coffee-Kahlua mixture.
In a medium-sized bowl, beat the mascarpone cheese, brown sugar, and granulated sugar together until completely smooth.
In another medium-sized bowl, beat the whipped cream until stiff peaks form.
Gently fold the whipped cream into the mascarpone cheese mixture.
Dip each ladyfinger into the coffee-Kahlua mixture, one at a time, and place them at the bottom of an 8-inch square pan. Do not soak them for too long or they will fall apart.
Repeat the previous step until all 10 ladyfingers have been used to cover the bottom of the square pan. You may need to break some of the ladyfingers to fit.
Spread half of the mascarpone cheese mixture on top of the ladyfingers.
Repeat steps 5-7 with the remaining 10 ladyfingers and mascarpone cheese mixture to create a second layer.
Sprinkle cocoa powder on top of the tiramisu to create a fine layer.
Refrigerate the tiramisu for 1-5 hours to allow the flavors to mix together.