In a medium heatproof bowl, whisk the sugar and egg yolks until smooth. In a medium saucepan, combine the cream and vanilla seeds and bring to a simmer. While whisking, gradually pour half of the hot cream into the egg yolk mixture. Gradually whisk the mixture back into the saucepan. Return the saucepan to the heat and cook, while whisking, until it reaches 185°F. Remove from the heat and stir in the gelatin until dissolved. Let the mixture cool to 104°F and whisk in the mascarpone. Pour the mixture through a fine-meshed sieve into the prepared loaf pan. Transfer to the freezer and chill until solid. Turn the parfait onto a cutting board, peel off the acetate paper, and cut into at least eighteen 1¼-inch squares. Reserve, frozen.