Note: Line an 8 by 8-inch baking pan aluminum foil, leaving extra to hang over the edge (this will make it easier to serve later). Spray with Pam or another release agent.
Make the cookie base. Put oven rack in middle position and preheat oven to 350°F.
Melt butter and chocolate with brown sugar in a heavy saucepan over moderate heat, stirring occasionally, until smooth. Remove from heat. Whisk in eggs and vanilla until combined. Whisk in flour, cocoa, and salt until just combined.
Spread batter evenly in baking pan and bake until set and a wooden pick inserted in center comes out with moist crumbs adhering, about 12 minutes. Cool completely in pan on a rack, about 1 1/2 hours.
Make the filling: Combine cream cheese, hazelnut-chocolate spread, and vanilla; beat with a mixer at medium speed until smooth. Gradually add powdered sugar, beating until just blended. Spread over the cookie and freeze.
Make the ganache: Put the chocolate and Irish Cream liqueur in the top of the double boiler, and set over the barely simmering water, then stir occasionally until melted and smooth. Remove from the heat and let cool slightly. The ganache should have a sheen to it, and hold together well. When cool but still pourable, spread the white chocolate layer over the chilled filling layer and smooth over. You can press additional nuts into the top at this point for decoration. Freeze for 30 minutes.
Serve: Remove from the freezer and let sit at room temperature for 5 minutes. Pull out of the pan and transfer to a cutting surface. Cut into 1-inch squares and serve cool or at room temperature.