Preheat the oven to 400° and position racks in the upper and lower thirds. Line 2 baking sheets with parchment paper.
In a large, wide bowl, using a large rubber spatula or a handheld electric mixer, mix the confectioners' sugar and the almond flour with 1 of the egg whites until evenly moistened.
In a small saucepan, combine the 1/2 cup of granulated sugar with the water; bring to a boil. Using a moistened pastry brush, wash down any crystals from the side of the pan. Boil over high heat until the syrup reaches 240° on a candy thermometer.
In another large bowl, using clean beaters, beat the remaining 2 egg whites at medium-high speed until soft peaks form. With the machine at high speed, carefully drizzle the hot syrup over the whites; beat until firm and glossy. Beat in the food coloring until the meringue is bright yellow.
Stir one-fourth of the meringue into the almond mixture. Using a rubber spatula, fold in the remaining meringue. Transfer the meringue to a pastry bag fitted with a plain 1/2-inch tip; pipe onto the prepared baking sheets in 1 1/2-inch mounds, spacing them about 1 inch apart. Tap the sheet and let dry for 15 minutes.
Transfer the meringues to the oven and immediately turn off the heat. Bake the meringues for 5 minutes. Turn the oven on to 400° again and bake for 8 minutes, until the meringues are puffed and the tops are firm and glossy. Transfer the baking sheets to racks; let cool completely.
Meanwhile, in a small saucepan, whisk the remaining 3 tablespoons of granulated sugar with the cornstarch. Whisk in the passion fruit puree and the cream. Whisk over moderate heat until thick, about 4 minutes. Remove from the heat; whisk in the white chocolate until melted. Let cool slightly, then whisk in the butter. Let cool completely.
Carefully peel the meringues off of the parchment paper. Spoon the filling into a pastry bag fitted with a 1/4-inch tip. Alternatively, use a resealable plastic bag and snip off a corner. Pipe the filling onto the flat sides of half of the meringues. Top with the remaining meringues and serve.
The macarons can be stored in an airtight container for up to 3 days.
Almond flour (finely ground blanched almonds) is available at supermarkets.