01 - Combine the dark chocolate and butter in a heatproof bowl set over simmering water. Stir gently until fully melted and smooth. Remove from heat and let cool for about 5 minutes.
02 - In a mixing bowl, whisk the egg yolks with half of the granulated sugar until the mixture turns pale yellow and becomes thick and creamy, approximately 2 to 3 minutes.
03 - In a separate clean bowl, whip the egg whites with a pinch of salt until soft peaks begin to form. Gradually add the remaining sugar while continuing to whip until stiff, glossy peaks hold their shape.
04 - Fold the egg yolk mixture into the slightly cooled chocolate until evenly combined. Gently fold in the whipped egg whites in three additions, using a spatula to preserve as much air as possible. Refrigerate the mousse for at least 30 minutes.
05 - In a chilled bowl, beat the mascarpone cheese, cold heavy cream, and powdered sugar together until smooth, creamy, and spreadable. Set aside.
06 - Pour the cooled strong coffee and coffee liqueur into a shallow dish and stir to combine.
07 - Quickly dip each ladyfinger into the coffee mixture, just long enough to moisten without saturating. Arrange the dipped ladyfingers in a single layer covering the bottom of an approximately 8x8 inch rectangular serving dish.
08 - Spread half of the mascarpone cream evenly over the ladyfinger base. Spoon half of the chocolate mousse over the mascarpone layer and smooth into an even surface.
09 - Arrange a second layer of coffee-dipped ladyfingers on top. Spread the remaining mascarpone cream, then finish with the rest of the chocolate mousse, smoothing the top evenly.
10 - Cover the dish tightly and refrigerate for at least 4 hours, preferably overnight for the best texture and flavor development. Just before serving, dust the top generously with cocoa powder using a fine sieve.