Make the Chestnut Shortcrust Pastry:
Pulse all-purpose flour, chestnut flour, powdered sugar, and salt in a food processor to combine.
Add cold butter cubes and pulse until mixture resembles coarse breadcrumbs with some pea-sized pieces remaining.
Add egg yolk and 1 tablespoon ice water. Pulse just until dough begins to clump. Add remaining water only if needed—chestnut flour absorbs less liquid than wheat flour.
Turn dough onto a work surface and bring together into a disc. Wrap tightly in plastic and refrigerate at least 1 hour.
Roll dough to ⅛-inch thickness. Use rolling pin guides or wooden dowels to ensure even thickness throughout.
Cut eight 5-inch circles for 3½-inch tartlet pans. Gently press dough into pans, trimming excess. Use the same technique for each—press from center outward, ensuring even thickness on sides. Prick bottoms with a fork.
Refrigerate lined pans 30 minutes. Meanwhile, preheat oven to 375°F (190°C).
Line each tartlet with parchment and fill with pie weights or dried beans. Blind bake 15 minutes, remove weights, and bake 5-8 minutes more until golden and dry. Cool completely.

Make the Blackcurrant Compote:
Combine blackcurrants, sugar, and lemon juice in a saucepan over medium heat.
Cook, stirring occasionally, until berries burst and mixture thickens to jam consistency, 10-12 minutes.
Remove from heat and stir in cassis if using. Reserve ¼ cup for the crema and meringue; let remainder cool completely.
For purée: press reserved portion through a fine-mesh sieve to remove seeds.

Make the Blackcurrant Crema:
Heat cream and milk in a saucepan until steaming.
In a bowl, whisk egg yolks, sugar, and cornstarch until smooth and pale.
Slowly pour hot cream mixture into yolks, whisking constantly to temper.
Return to saucepan and cook over medium heat, whisking constantly, until mixture thickens and just begins to bubble, 3-4 minutes.
Remove from heat. Whisk in blackcurrant purée, cassis if using, and salt.
Strain through a fine-mesh sieve into a bowl. Press plastic wrap directly onto surface and refrigerate until cold and set, at least 2 hours.

Make the Chestnut Cream:
In a stand mixer with paddle attachment, beat sweetened chestnut purée and butter until smooth and fluffy, 2-3 minutes.
Add unsweetened chestnut purée and mix until combined—this deepens the chestnut flavor per Prue's feedback.
In a separate bowl, whip heavy cream to soft peaks.
Fold whipped cream into chestnut mixture in two additions. Add rum, vanilla, and salt. Mix until smooth.
Transfer to a piping bag fitted with a Mont Blanc tip (multi-holed) or small round tip. Refrigerate until needed.

Assemble the Base Layers:
Spoon 1 tablespoon blackcurrant compote into each cooled tartlet shell and spread evenly.
Spoon or pipe blackcurrant crema over compote, filling shells about two-thirds full. Smooth tops.
Pipe chestnut cream in a mound over the crema. For traditional Mont Blanc style, use a multi-holed tip to create vermicelli-like strands in a nest shape. Pipe the same amount onto each tartlet—weigh if necessary for uniformity.
Refrigerate assembled tartlets while preparing meringue.

Make the Yuzu-Blackcurrant Meringue:
Combine egg whites, sugar, cream of tartar, and salt in a heatproof bowl. Set over a pan of simmering water (bowl should not touch water).
Whisk constantly until sugar dissolves and mixture reaches 160°F (71°C), 3-4 minutes.
Transfer to stand mixer with whisk attachment. Beat on high until stiff, glossy peaks form and meringue is cool, 8-10 minutes.
Beat in yuzu juice and zest.
Transfer about one-third of meringue to a small bowl. Fold in blackcurrant purée and purple coloring if using—this creates a swirled effect.
Spoon plain and blackcurrant meringues side by side into a piping bag fitted with a large star or round tip. The two colors will create natural marbling when piped.
Pipe meringue onto each tartlet in decorative swirls or peaks, covering the chestnut cream. Use the same piping motion for each—practice on parchment first if needed.

Torch the Meringue:
Hold blowtorch 3-4 inches from meringue surface. Move in slow, steady, overlapping strokes—never hold still in one spot.
Work methodically: start at one edge, move across in horizontal passes, then return to ensure even coverage. The entire surface should be touched by flame.
Aim for golden-brown peaks with slightly darker tips. Check each tartlet from all angles before moving to the next.
Alternatively: Place under a hot broiler for 30-60 seconds, watching constantly and rotating for even browning.

Finish and Serve:
Garnish with fresh blackcurrants, candied chestnut pieces, chestnut shavings, and gold leaf if desired.
Serve within 2-3 hours for best meringue texture.