In a small bowl, mix together the maple syrup and vinegar. Set aside. Heat a small skillet over high heat and add the peppercorns, fennel seeds, and coriander seeds. Toast until fragrant, swirling the spices occasionally so they toast evenly, 30 to 45 seconds. Remove from the heat and transfer the seeds to a mortar and pestle. Gently crack the seeds until coarse, and stir into the maple-vinegar syrup. Cover with plastic wrap and refrigerate for at least 2 hours, and preferably overnight.
Halve the vanilla bean lengthwise and scrape the seeds into a small bowl. Add the crème fraîche and confectioners’ sugar, and whisk to combine. Cover with plastic wrap and refrigerate until ready to use.
Position a rack in the upper third of the oven and preheat the broiler. Brush the flesh of the peach halves with the melted butter and place them in a baking dish or on a baking sheet, skin-side down. Broil the peaches, watching carefully so they don’t burn, until the tops start to caramelize and the juices bubble, 5 to 6 minutes. Transfer to a serving dish.
To serve, top each peach half with some of the chilled crème fraîche and a generous drizzle of the spiced maple-vinegar syrup. Serve immediately.
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