For the Sponge: In the bowl of a stand mixer fitted with a paddle attachment, add flour, yeast, and 349 grams cold water. Mix until well combined. Transfer to a large airtight container and refrigerate overnight.

For the Dough: In the bowl of a stand mixer fitted with a paddle attachment, add milk, butter, Sponge, and 458 grams ice water. Mix until just incorporated then add flours, sugar, salt, yeast, and pâte fermentée. Mix 4 minutes on low speed. Increase speed slightly and mix an additional 6 minutes. Transfer to a lightly floured work surface and form mixture into a rectangle. Wrap tightly with plastic wrap and freeze overnight.

For the Vanilla Pastry Cream: In a large pot over medium-high flame, heat milk and vanilla until steaming. Reduce heat and keep warm. In a mixing bowl, whisk to combine egg yolks and sugar. Add cornstarch and whisk until well incorporated. Temper egg mixture with hot milk mixture. Add yolk mixture to the pot, whisking constantly. Cook until mixture has thickened to desired consistency. Remove from heat. Working in batches, whisk in butter. Strain onto parchment-lined sheet trays. Let cool. Once cooled, transfer mixture to a piping bag and refrigerate.

For the Croissants: On a work surface, laminate Dough with butter. Using a sheeter, roll out Dough until 3½-millimeters thick. Return Dough to work surface. Using a 4-inch circle cutter, punch out rounds. Transfer rounds to a sheet tray and freeze until chilled.

To Assemble and Serve: Heat oven to 425°F. On a work surface, carefully place Croissant rounds into desired molds. Freeze until set. Fill each frozen Croissant shell with 75 grams Vanilla Pastry Cream. Transfer to a sheet tray and bake 20 minutes, or until tops are browned. Remove Croissants from molds, then brush with vanilla simple syrup.