Beat brown sugar and butter in large bowl on low to medium speed of mixer until combined.
Add eggs, one at a time, beating well after each addition.
Beat in coffee liqueur or coffee and vanilla.
Add flour mixture and sour cream alternately to sugar mixture, beating after each addition just until combined.
Stir in boiling water until blended.
Pour into prepared pans.
Bake 30 to 35 minutes or until wooden pick inserted near centers comes out clean.
Cool in pans on wire racks 10 minutes; remove from pans to wire racks.
Split each cake layer horizontally to make four layers total.
Place one layer on serving plate; spread with one-third of the mousse (about 1 cup).
Repeat layering with two of the remaining layers and remaining mousse.
Place remaining cake layer on top.
Place HERSHEY'S Semi-Sweet Chocolate Chips in a food processor bowl; cover and process until finely ground.
Mix 1/3 cup whipping cream and 3 tablespoons granulated sugar in 1-quart saucepan.
Cook over medium heat, stirring constantly, until sugar is dissolved and mixture is just boiling.
With food processor running, pour hot cream through feed tube, processing 10 to 20 seconds or until chocolate is completely melted.
Scrape side of food processor bowl.
Add coffee liqueur or strong coffee and vanilla extract through feed tube; process 10 to 20 seconds or until smooth.
Pour into large bowl; cool about 10 minutes or until mixture is room temperature.
Beat 1 cup whipping cream in chilled medium bowl on high speed of mixer just until soft peaks form.
Fold whipped cream into chocolate mixture.
Cover; refrigerate at least 30 minutes.
Stir together sifted powdered sugar and HERSHEY'S SPECIAL DARK Cocoa in medium bowl.
Add whipping cream and vanilla extract.
Beat on low speed of mixer until stiff peaks form, scraping side of bowl constantly. (Mixture will be very stiff.) By hand, stir in milk, one tablespoon at a time, to make desired spreading consistency.
Frost cake top and sides.
Cover; refrigerate at least 2 hours before serving.