Preheat oven to 350°. Arrange pecans in a single layer on a baking sheet, and bake 5 to 7 minutes or until lightly toasted and fragrant.
Stir together coffee liqueur and coffee.
Coarsely chop 1 chocolate bar. Microwave coarsely chopped chocolate and butter in a large microwave-safe bowl at MEDIUM (50% power) 1 to 1 1/2 minutes or until melted and smooth, stirring at 30-second intervals. Whisk in 2 cups sugar and eggs, whisking until blended; stir in flour, stirring just until blended. Spoon batter into a lightly greased 11- x 7-inch pan.
Bake at 350° for 35 minutes or until center is set. Remove from oven, and cool in pan on a wire rack 10 minutes. Pierce brownie multiple times using the tines of a fork. Pour coffee mixture over brownie. Let cool on wire rack 1 hour or until completely cool.
Whisk together remaining 1/4 cup sugar, mascarpone cheese, and vanilla in a large bowl. Beat whipping cream at medium speed with an electric mixer until stiff peaks form. Fold whipped cream into mascarpone mixture.
Crumble half of brownies, and divide evenly among 6 (8-oz.) glasses. Spoon half of mascarpone cheese mixture over brownies. Repeat procedure with remaining brownies and mascarpone cheese mixture.
Chop remaining half of chocolate bar into thin shreds. Sprinkle chocolate and pecans over mascarpone cheese mixture. Serve immediately, or cover and chill up to 24 hours.
Mascarpone gives the topping a special flavor, but an equal amount of softened cream cheese may be substituted. Almond-flavored liqueur makes a delicious substitute for coffee liqueur.