Make lavender custard. Heat milk in a medium saucepan until almost boiling. Meanwhile, whisk egg yolks, sugar, cornflour and lavender flowers in a large bowl, gradually whisk in hot milk. Return mixture to pan and whisk continuously over medium heat for 7 minutes or until mixture boils and thickens. Remove from heat and continue to whisk a further minute. Strain into a medium bowl, cover surface with plastic wrap and refrigerate until cool. Just before using, beat cream in a small bowl with an electric mixer until soft peaks form, then fold into lavender custard.

Preheat oven to 160°C/325°F. Grease three large oven trays, line with baking paper.

Brush a sheet of fillo with some of the melted butter and scatter with scant ¾ teaspoon of caster sugar. Repeat layering five times to form a six layer stack. Using a sharp knife and ruler as a guide, cut stack into two 12cm x 28cm (4¾-inch x 11¼-inch) rectangles, discard trimmings.

Repeat step 3 two more times. You will have six equally-sized fillo stacks in total.

Place two fillo stacks, side by side, on each oven tray, cover with a sheet of baking paper. Stack trays on top of each other. Place a fourth oven tray on top of the stack, weight with an ovenproof dish.

Bake stacked trays for 15 minutes. Carefully remove from oven, remove dish, un-stack trays. Return trays to oven, on separate shelves, bake, uncovered, a further 10 minutes or until golden. (For even cooking, rotate trays during cooking. If pastry starts to puff, press it down with a spatula.) Cool pastry on trays. Transfer to an airtight container until ready to assemble.

To assemble mille feuille, place a fillo rectangle on a platter; spread with one-fifth of the lavender custard. Repeat layering, finishing with a fillo rectangle. Dust with icing sugar and decorate with lavender flowers, if you like.