Spread the bottom layer of the cake with about 1/4 cup apricot jam. Shape almond paste into a flat disk and place on a sheet of plastic wrap; cover with a second sheet of plastic and roll out into a 9-inch round. Peel off top layer of plastic and invert almond paste onto bottom layer of cake; peel off remaining plastic wrap and trim off any uneven edges. Spread bottom of top cake layer with about 1/4 cup apricot jam; set layer, jam side down, on almond paste. Warm 3/4 cup apricot jam in a microwave oven or over low heat on the stove, then pour through a fine wire strainer; you should have about 1/2 cup. Beat into buttercream; use apricot buttercream to frost cake. Garnish top or sides with sliced almonds.