Make the lady fingers: Preheat oven to 375°F. Line baking sheets with parchment paper. Whip 3 egg whites until stiff peaks form, gradually adding 2 tablespoons powdered sugar. In separate bowl, beat 3 egg yolks with 1/4 cup sugar until pale. Fold in 1/2 cup flour and baking powder, then fold in egg whites. Pipe 3-inch strips onto baking sheets. Dust with powdered sugar and bake 8-10 minutes until golden.
Prepare the mascarpone cream: In a large bowl, beat 6 egg yolks with 1/2 cup sugar until pale and thick, about 3-4 minutes. Gently fold in mascarpone cheese and vanilla extract until smooth.
Whip the egg whites: In a clean bowl, beat 6 egg whites with remaining 1/4 cup sugar and pinch of salt until stiff peaks form. Gently fold into mascarpone mixture in three batches.
Make coffee mixture: Combine cooled espresso with coffee liqueur or rum in a shallow bowl. Stir well.
Assemble the tiramisu: Spread a thin layer of cream in bottom of 9x9 inch dish. Quickly dip lady fingers in coffee mixture (1-2 seconds per side) and arrange in single layer over cream. Spread half the remaining cream over lady fingers. Repeat with another layer of dipped lady fingers and remaining cream.
Chill and serve: Cover and refrigerate at least 6 hours or overnight. Before serving, dust generously with cocoa powder using a fine-mesh sieve. Cut into slices with a sharp knife dipped in hot water.