In a small saucepan, combine 1 tablespoon instant coffee granules with 1/4 cup water. Heat gently over low heat, stirring until dissolved. Remove from heat and let cool slightly. Stir into 2 cups brewed coffee. Set aside to cool completely.
In a large mixing bowl, pour 1 cup heavy cream, 1/2 cup sugar, and 1 teaspoon vanilla extract. Whip with electric mixer or hand whisk until soft peaks form.
In another bowl, gently fold together 1 cup mascarpone cheese and 1/2 cup cocoa powder until smooth and well combined.
Carefully fold whipped cream into the mascarpone-cocoa mixture using a spatula, keeping the mixture airy.
Quickly dip each ladyfinger cookie into the cooled coffee mixture for 1-2 seconds per side. Lay a layer of dipped ladyfingers in your dish. Spread half of the creamy mascarpone mixture over the cookies. Repeat with another layer of dipped ladyfingers and finish with the remaining creamy mixture on top.
Cover the dessert with plastic wrap and refrigerate for at least 4 hours or overnight. Before serving, dust the top generously with cocoa powder.