Preheat oven to 325F (160C).

Mix flour, cocoa powder, salt, and sugar in a bowl and stir to blend.

In a separate bowl, beat egg yolks until thick and creamy.

Add lemon juice and water to the egg yolks and blend.

Pour the egg yolk mixture into the dry mixture and mix well.

Pour the batter into an ungreased 9-inch tube pan.

Bake in preheated oven for 35 to 40 minutes.

Cool the cake completely.

Cut the cake crosswise into 3 layers.

Prepare the mocha filling by combining pudding mix and instant coffee powder in a saucepan.

Blend in milk and cook on low to medium heat until thick, stirring constantly.

Remove from heat and cool the filling.

Cream the butter, add to the cooled pudding mixture, and beat until smooth and fluffy.

Blend in vanilla extract and powdered sugar.

Spread the mocha filling between the cake layers.

Dust with additional powdered sugar if desired.