Preheat oven to 200C. Roll out pastry on a lightly floured surface to 3mm thick and refrigerate to rest (30 minutes). Cut out two 12cm x 25cm rectangles, prick with a fork and place each on a baking tray lined with baking paper, then freeze until firm (30 minutes).
Meanwhile, for almond praline, roast almonds on an oven tray until fragrant and toasted (6-7 minutes), then cool slightly, chop and set aside. Stir sugar and 60ml water in a small saucepan over medium-high heat until sugar dissolves, bring to the boil and cook without stirring, swirling pan occasionally and brushing down sides with a wet pastry brush if crystals form, until dark caramel (4-5 minutes). Remove from heat, quickly mix in almonds, then pour onto a lightly oiled oven tray and set aside to harden (30 minutes). Break into shards and store in an airtight container until required. Store praline for up to a week in an airtight container. Just before serving, finely chop half the praline.
Brush pastry lightly with eggwash, then place 4 small 2cm-high cookie cutters in the corners of each tray, cover with a piece of baking paper, then another baking tray (this helps the pastry rise evenly) and bake, swapping trays halfway through to cook evenly, until dark golden and crisp (20-25 minutes). Cool on trays for 15 minutes, then cool completely on wire racks.
For chocolate crème, heat chocolate and brandy in a bowl over a saucepan of simmering water, stirring occasionally, until smooth, then transfer to an electric mixer, add cream and whisk until soft peaks form (4-5 minutes). Refrigerate to firm up slightly (5-10 minutes).
Halve each puff pastry rectangle lengthways with a large serrated knife and place one on a serving platter to form the base of the millefeuille, spread with a third of the chocolate crème, scatter with a little chopped praline, then sandwich with another piece of puff pastry. Repeat layering, finishing with a piece of pastry. Dust with icing sugar and crumble some of the remaining praline on top. Cut into slices with a large serrated knife and serve immediately.