Cook frozen mixed
berries and 6 tablespoons sugar in heavy medium
saucepan over medium heat until mixture resembles
jam and is reduced to 1 cup, stirring frequently,
about 15 minutes. Cool jam mixture.
Strain syrup from
thawed raspberries through sieve set over bowl,
pressing gently on solids. Discard solids. Add
raspberry liqueur to raspberry syrup in bowl.
Using sharp knife, trim 1 biscuit to 3-inch
(about) length. Quickly dip biscuit into syrup,
turning to coat lightly. Place rounded end up
and sugared side against side of 9-inch-diameter
springform pan with 2 3/4-inch-high sides. Repeat
with as many biscuits as necessary to cover
sides of pan. Dip more biscuits in syrup and
arrange on bottom of pan, covering completely
and trimming to fit.
In bowl, whisk
mascarpone with 6 tablespoons sugar and vanilla
to blend. Set aside. Thinly slice enough strawberries
to measure 1/2 cup. Gently spread half of jam
mixture over biscuits in bottom of pan. Spoon
half of mascarpone mixture over; smooth top.
Sprinkle with sliced strawberries, 1/2 cup fresh
raspberries and 1/2 cup blueberries. Dip more
biscuits into syrup; arrange over fruit in pan,
covering completely and trimming to fit. Gently
spread remaining jam mixture over biscuits.
Spoon remaining mascarpone mixture over; smooth
top. Cover; chill at least 4 hours or overnight.
Release pan sides.
Transfer cake to platter. Arrange remaining
fresh berries decoratively atop cake and serve.