Preheat oven to 350 degrees F. Butter the bottom and sides of an 8-inch square baking pan. Line the bottom of the pan with parchment or waxed paper and coat the paper with butter.
Place the butter and the chocolate in the top of a double boiler over hot, not simmering water. Stir the mixture until it is completely melted and smooth. Remove the top part of the double boiler from the bottom and allow the mixture to cool.
In a medium bowl, whisk together the eggs and sugar. Whisk in the hazelnut liqueur and the cooled chocolate mixture. With a rubber spatula, stir in the flour just until blended. Stir in the hazelnuts. Scrape the batter the prepared pan. Bake for 25 to 30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs clinging to it. Cool the brownies. Run a paring knife around the edges of the pan and carefully invert the brownies onto a flat surface. Peel off the paper and then re-invert the brownies onto a cooling rack. Cool completely.
Place the chocolate in a medium bowl. Sift the cocoa powder over the chocolate. In a small saucepan bring the cream to a boil. Pour the hot cream over the chocolate and gently whisk until smooth. Stir in the hazelnut liqueur, and allow the mixture to cool at room temperature until it is spreadable, about 1 hour. Using a metal spatula, spread the frosting over the top of the brownies. Using a long serrated knife, trim approximately 1/4 to 1/2-inch off each side. Cut the brownies into 9 squares. Store in an airtight container.
In the top of a double boiler over hot, not simmering water, melt the chocolate with 1/4-cup heavy cream, stirring frequently. Let cool to room temperature, stirring once or twice.
In an electric mixer, using the whisk attachment beat the remaining 1-cup cream just until soft mounds barely start to form. Stir 2 tablespoons of the cooled white chocolate mixture into the whipped cream to lighten it. Gently fold in the remaining white chocolate mixture, just until combined. Scrape the cream into a pastry bag fitted with a large star tip (Ateco #7). Use to garnish brownies when plating.
Spread the tempered bittersweet chocolate over a polka-dot transfer sheet cut into a triangle (3 inches high). Allow to set up and cool. Remove chocolate and transfer from triangle. Use to garnish brownies and white chocolate whipped cream when plating.
Source for polka dot transfer sheets: NY Cake & Baking Distributor, 56 West 22nd Street, NY, NY 10010, tel. 212-675, CAKE or 1-800-94-CAKE-9.;