In a large bowl, whip 2 cups heavy cream, 5 tbsp granulated sugar and 1/2 tsp vanilla extract until combined. Add 1 lb mascarpone cheese and continue whipping to medium peaks. Set aside.
In another bowl, stir 1 cup espresso with 2 tbsp kahlua.
To assemble (see video): dip both sides of each savoiardi/ladyfinger cookie in your coffee mixture. Place on the bottom of a deep baking dish (I used a 3-quart dish) or a 9x13 baking pan. Cut the cookies if needed to fit your dish exactly.
Spread half of the mascarpone mixture on top of the cookies. Sprinkle the chopped chocolate on top of that.
Repeat with another layer of cookies and mascarpone cream. Chill 6-8 hours, preferably overnight. Sprinkle with cocoa powder before serving.