In a large bowl, cream butter and sugar until light and fluffy. Add chocolate; mix well. Add egg yolks, one at a time, beating well after each addition. Add milk and vanilla; mix well.
Combine the flour, baking powder and salt; stir into chocolate mixture just until blended.
In a small bowl, beat egg whites until soft peaks form. Fold into batter. Pour into a greased and floured 9-in. spring form pan. Place on a baking sheet. Bake at 350° for 25-30 minutes or until a toothpick inserted near the center comes out clean. Cool completely on a wire rack.
In a small saucepan, combine chocolate chips, 3/4 cup cream, sugar, butter and coffee granules. Cook and stir over low heat until sugar is dissolved and chips are melted. Stir a small amount of hot liquid into egg yolks; return all to the pan. Cook and stir for 2 minutes or until mixture is slightly thickened. Remove from the heat; stir in vanilla.
Sprinkle gelatin over cold water; let stand for 1 minute. Add a small amount of hot liquid to gelatin; stir until gelatin is dissolved. Stir into chocolate mixture. Pour into a large bowl. Set the bowl in a larger bowl of ice water; stir occasionally until thickened, about 30 minutes.
Remove sides of spring form pan. Cut a hole in the corner of a pastry or plastic bag; fill bag with 1/4 cup mousse. Set aside for garnish. Spoon half of the remaining mousse over brownie layer, spreading evenly over top and around sides.
In another large bowl, beat the remaining cream until soft peaks form. Add a third of the whipped cream to the remaining mousse; beat on low speed. Fold in remaining whipped cream. Spread over truffle mousse layer.
Gently press cookies into sides of dessert. Pipe reserved mousse on top. Cover and refrigerate for at least 4 hours or overnight. Remove from the refrigerator 5 minutes before cutting. Yield: 12-16 servings.