Heat oven to 350°F. Lightly spray 9-inch tart pan with removable bottom with cooking spray. In medium bowl, mix cookie crumbs and 1/4 cup sugar. Stir in 1/4 cup butter. Press crumb mixture in bottom of pan.
Bake 10 minutes. Cool in pan on cooling rack while making white chocolate layer.
Cut 1 square of white chocolate baking bar in half; reserve half. Finely chop other half of square and remaining baking bar. Set aside. In small saucepan, cook all remaining white chocolate layer ingredients over medium-low heat until butter is melted and mixture is simmering, stirring occasionally. Remove from heat. Stir in chopped white chocolate until smooth. Spoon into crust-lined pan; spread gently. Refrigerate while preparing dark chocolate layer.
In medium saucepan, cook 3/4 cup whipping cream and 1 tablespoon butter over medium-low heat until butter is melted and mixture is simmering, stirring occasionally. Remove from heat. Stir in semisweet chocolate until smooth. Stir in 2 tablespoons liqueur. Spoon over white chocolate layer; spread gently. Refrigerate about 2 hours or until set.
About 1 hour before serving, grate reserved half square of white chocolate over surface of tart. Remove side of pan; let tart stand at room temperature 1 hour.
To serve, cut tart into wedges; place on individual serving plates. Poke hole with toothpick in shortest side of each wedge of tart. Insert chocolate stick piece into each hole to resemble stem of tree.