Put oven rack in middle position and preheat oven to 350F.

Cook shallots in olive oil in a 10-inch heavy skillet over moderate heat, stirring occasionally until softened, about 4 minutes. Add garlic and cook, stirring, 1 minute.

Remove from heat and carefully add Cognac (use caution; if Cognac ignites,

shake skillet), then boil until reduced to about 2 tablespoons, 1 to 2 minutes.

Transfer to a blender and add livers and yolks, then purée until smooth.

Add milk, flour, salt, pepper, nutmeg, and allspice and blend until combined.

Pour into a crock, skimming off any foam.

Put crock in a larger baking pan and bake in a water bath until mousse is just set and a small sharp knife inserted in center comes out clean, about 55 minutes.

Melt butter in a small saucepan over low heat, then remove from heat and let stand 3 minutes.

Arrange bay leaves decoratively on top of mousse.

Skim froth from butter, then spoon enough clarified butter over mousse to cover its surface, leaving milky solids in bottom of saucepan.

Chill mousse completely, uncovered, about 4 hours.

Bring to room temperature about 1 hour before serving