To make the pastry cream, heat the milk to a boil in a heavy medium saucepan over medium heat, being careful not to scorch. Meanwhile, whisk the sugar, egg yolks, and cornstarch in a medium mixing bowl until smooth.
Whisk 1/2 cup of the hot milk into the egg mixture in a slow, steady stream. Then whisk the egg mixture into the saucepan of hot milk. Stir constantly over medium-low heat until the mixture thickens enough to coat the back of a spoon and reaches 170°F on an instant-read thermometer, about 5 minutes. Remove from the heat and stir in the pineapple, butter, and vanilla. Pour into a bowl.
Press a piece of plastic wrap directly onto the pastry cream to prevent a skin from forming. Let cool completely in the refrigerator.
To make the berry filling, gently stir the strawberries, blueberries, raspberries, sugar, and Grand Marnier in a mixing bowl, being careful not to crush the berries. Cover and refrigerate until chilled, at least 1 hour.
To make the wafers, in a dry medium skillet, cook the sesame seeds over medium heat, stirring often, until fragrant and golden, about 3 minutes. Immediately transfer to a plate and let cool completely.
Preheat the oven to 375°F. Line two baking sheets with parchment paper.
Combine the flour, sugar, and salt in a medium bowl. Add the butter and rub between your thumbs and fingertips until the mixture resembles coarse crumbs. Add the sesame seeds and yolks and mix to moisten. One tablespoon at a time, work in the ice water with your fingers just until the dough comes together, being careful not to overmix.
Roll out the dough on a lightly floured work surface until 1/8 inch thick. With a 4-inch round cookie cutter or the bottom of a coffee can, cut the dough into 12 rounds. Arrange the rounds on the prepared baking sheets. Cover the wafers with another sheet of parchment paper.
Bake until the wafers are lightly golden, about 15 minutes. Cool the wafers on the pans for a few minutes, then gently transfer the wafers to wire racks and let cool completely.
Whip the heavy cream with the sugar in a chilled medium bowl until soft peaks form.
Spread 8 of the wafers with 1/4 cup pastry cream each. Place 1 wafer, pastry cream side up, on each of four serving plates, and spoon 1/4 cup of the fruit over each. Top each with another wafer, pastry cream side up, and spoon another 1/4 cup of the fruit over each. Top each with a plain wafer, then with a generous spoonful of the whipped cream. Garnish each with a mint sprig and serve immediately.
Yield: 4 servings