Combine the mascarpone, heavy cream, sugar, vanilla extract and vanilla seeds in the bowl of a stand mixer and whip until soft peaks form. Using a rubber spatula, scrape the mixture into a pastry bag fitted with a star tip.
Stand the strawberries upright on a platter. If the bottoms are too wobbly to stand, slice off the green stems. With a small sharp knife, cut an "X" in each strawberry, cutting almost to the bottom. (The cut should be such that the strawberry will open up but not fall apart.) With your fingertips, gently spread each strawberry apart to make "petals."
With a spoon, carefully fill each strawberry with a dollop of cream. Serve immediately.