STEP 4: Raspberry Jam. Over medium heat, mix raspberries, sugar, lemon juice, and salt. Mash. Run through a sieve. Add cornstarch and jam to hot pan until thickened. Cool completely.
STEP 5: Raspberry Frosting. Beat butter and cream cheese on high for 3 minutes. Add in sifted powdered sugar and raspberry mixture. Mix in vanilla, salt, and heavy cream until combined.
STEP 6: Assemble. Cut off cake domes. Place 1st layer down onto a small amount of frosting. Spread 1 cup of frosting over the cake. Pipe a border around the edge. Spread jam on top.
STEP 7: Place last layer upside down. Frost the cake with a light layer and freeze. Frost the rest of the cake and decorate with fresh raspberries. Freeze before serving.