Place the cooked chestnuts, sugar, water, vanilla seeds and pod, and a pinch of salt in a small saucepan. Warm over medium-low heat until sugar dissolves and mixture is gently simmering, about 5–7 minutes. Stir occasionally to prevent sticking.

Blend purée

Remove the vanilla pod. Transfer the warm chestnut mixture to a blender or food processor. Add the rum (if using). Purée until completely smooth, 1–2 minutes. If the purée seems too thick or grainy, add up to 30–60 ml warm water, 1 tbsp at a time, and blend to reach a pipeable, smooth consistency.

Refine

For ultra-smooth texture, pass purée through a fine-mesh sieve or tamis, pressing with the back of a spoon. Stir in butter or 15 g mascarpone (optional) until incorporated. Taste and adjust sweetness if needed by stirring in a little more sugar (up to 15 g). Cool completely to room temperature; refrigerate briefly if needed until pipeable but not stiff (about 20–30 minutes).

Meringue Bases

Prepare oven and piping

Preheat oven to 100°C (210°F). Line two baking sheets with parchment paper. Fit a piping bag with a plain round tip (8–10 mm) or use a spoon to shape discs about 6–7 cm (2.5–3 in) wide.

Make meringue

In a perfectly clean, dry bowl, whisk egg whites until foamy. Add cream of tartar and continue whisking to soft peaks. Gradually add the 150 g granulated sugar, 1 tablespoon at a time, whisking until glossy, stiff peaks form and sugar is dissolved. Whisk in cornstarch and 1 tsp vanilla extract on low speed until combined.

Pipe or spoon bases

Pipe or spoon 6–8 round meringue bases (depending on size) onto the prepared sheets, making a slight well in each center to hold whipped cream. Smooth edges as needed.

Bake and cool

Bake at 100°C (210°F) for 70–90 minutes until dry to the touch and easily lift off paper. Turn off oven and leave meringues inside with door slightly ajar for 30 minutes to prevent cracking from sudden temperature change. Cool completely on racks.

Whipped Cream Filling

Chill equipment

Chill a mixing bowl and whisk (or beaters) in the refrigerator for 10–15 minutes to help cream whip quickly.

Whip cream

Pour chilled heavy cream into the cold bowl. Add powdered sugar and vanilla extract. Whip on medium-high speed until medium-stiff peaks form — firm enough to hold shape but still light. Do not overwhip. Keep chilled until assembly.

Assembly & Garnish

Fill meringue bases

Place a meringue base on a serving plate. Pipe or spoon a generous mound of whipped cream into the center, forming a rounded dome about 3–4 cm high. Repeat for each base.

Pipe chestnut vermicelli

Fit a piping bag with a Mont Blanc (multi-hole) tip or a small round tip (approx. 3–4 mm). Fill with the chestnut purée. Starting at the top center of the cream dome, pipe thin strands of purée in circular motions, working downward to cover the whipped cream completely and form the characteristic mountain shape. Continue piping until each mound is fully covered and you achieve a conical peak.

Garnish and finish

Dust lightly with cocoa powder. Top each Mont Blanc with a candied chestnut or a small piece of marron glacé if available, and a mint leaf for color. Serve immediately, or keep refrigerated for up to 6 hours (meringue will soften over time).