Melt the chocolate in a metal bowl placed over a pot of barely simmering water, stirring gently until melted. Remove the bowl from the heat.
Whisk the egg yolks, 2 tbsp of the sugar, the Irish Cream liqueur in another metal bowl, and place this over the same pot of water, this time at a full simmer. Whisk the mixture constantly until it has doubled in volume and holds a ribbon when the whisk is lifted. Whisk this into the melted milk chocolate and set aside to cool to room temperature.
Whip the cream to soft peaks and whisk in the vanilla and peppermint extracts. In a clean bowl, whip the egg whites and salt until foamy, then gradually add the remaining 2 Tbsp of sugar, whipping until the whites hold a soft peak.
Fold the whipped cream into the cooled chocolate base in 2 additions, then fold in the whipped whites. Spoon this into a 9-x-5-inch loaf pan that has been lined with plastic wrap or in the paper cones resting in glasses to keep them level and upright. Cover the top with plastic and freeze until firm, about 6 hours for the loaf pan and about 3 hours for the cones.
To serve, turn the semifreddo out onto a platter and dust with cocoa powder before slicing to serve from the loaf pan. For the paper cones, carefully peel away the paper and serve inverted on a plate dusted with cocoa, or as an accent on top of a tart or brownie.