This will prevent the pastry cream from developing a dry skin Place the sheet pan in the freezer to cool the cream rapidly and stop the growth of bacteria This should only take 15 minutes Once cool, transfer to the refrigerator and chill for 30 minutes To finish the mousseline, remove your pastry cream from the refrigerator and bring it to room temperature so that it is not too cold Make sure that your butter is nice and soft Both components should be around 60°F/16°C Place the pastry cream in the mixer fitted with the whisk attachment and whisk on high speed for 1 minute Scrape down the sides of the bowl with a rubber spatula and whip for another minute on high Add the soft butter and whip until the butter is homogenized and the mixture is glossy If the cream is grainy this indicates that the butter or the pastry cream is too cold To remedy this, place the mixing bowl in a bowl of hot water for 15 seconds, then return to the mixer and whip again for 15 seconds Repeat until the cream is very shiny Refrigerate for an hour or two, until ready to use BAKING THE PUFF PASTRY AND ASSEMBLING THE MILLE-FEUILLE Preheat the oven to 400°F/200°C with the rack positioned in the middle Place a wire rack on top of the puff pastry with the legs down; the rack should be ½ inch above the dough It’s okay if the legs have to sit on the dough; they will cause indentations, but these won’t matter Place the sheet pan in the hot oven and bake for 20 minutes ... BAKING THE PUFF PASTRY AND ASSEMBLING THE MILLE-FEUILLE Preheat the oven to 400°F/200°C with the rack positioned in the middle Place a wire rack on top of the puff pastry with the legs down; the. .. another minute on high Add the soft butter and whip until the butter is homogenized and the mixture is glossy If the cream is grainy this indicates that the butter or the pastry cream is too cold... around 60°F/16°C Place the pastry cream in the mixer fitted with the whisk attachment and whisk on high speed for 1 minute Scrape down the sides of the bowl with a rubber spatula and whip for another minute on high