Preheat your oven to 375°F (190°C). Lightly grease a baking dish large enough to hold the nectarines.

Wash the nectarines, slice them in half lengthwise, and remove the pits. Place the nectarine halves cut-side up in the prepared baking dish.

Brush the tops and cut surfaces of the nectarines generously with the melted butter. Sprinkle evenly with the brown sugar.

Bake for 20 to 25 minutes, or until the fruit is tender and slightly caramelized. While baking, prepare the sauce.

To make the sauce, combine the honey, water, and ground cinnamon in a small saucepan. Bring the mixture to a simmer over medium heat, stirring until the honey is fully dissolved (about 3 minutes). Remove from heat and stir in the vanilla extract, if using.

Once the nectarines are baked, carefully transfer two halves to each serving plate. Spoon the warm cinnamon honey sauce generously over the fruit before serving immediately.