Preheat the oven to 425°F (220°C) and line a roasting pan with parchment paper.

Halve and seed the kabocha squashes.

Soften the butter.

Rub 1 tablespoon of softened butter inside each squash half.

Press a sage leaf onto the butter in each squash half.

Season the squash halves with salt and freshly ground pepper.

Place the squash halves cut side down in the prepared roasting pan.

Add 1 cup of water to the pan.

Cover the pan tightly with aluminum foil.

Roast in the preheated oven for 1 hour, or until the squash is tender when pierced with a fork.

Remove the pan from the oven and let cool slightly.

Discard the sage leaves from the roasted squash.

Scoop the cooked squash flesh into a bowl.

In a small skillet, melt the remaining 4 tablespoons of butter over moderate heat.

Add the remaining 4 sage leaves to the melted butter.

Cook the sage leaves in the butter until the butter is lightly browned and the sage is crispy, about 5 minutes.

Transfer the fried sage leaves to a plate to drain.

Add the browned butter and lemon juice to the bowl with the squash.

Mash the squash mixture to a smooth puree using a potato masher or an immersion blender.

Season the puree with salt and freshly ground pepper to taste.

Transfer the kabocha squash puree to a serving bowl.

Drizzle the puree with aged balsamic vinegar.

Garnish with the fried sage leaves and serve immediately.