Preheat the oven to 400°F (200°C). Line a large baking sheet with parchment paper or lightly grease it. Using a sharp chef's knife, cut off the stem and blossom ends of the butternut squash, peel with a vegetable peeler, cut in half lengthwise, scoop out and discard the seeds, then cut the squash into 1-inch cubes for even roasting. Aim for uniform pieces so they cook at the same rate.

Make brown sugar glaze

In a small saucepan over low heat, combine the unsalted butter, brown sugar, and maple syrup. Stir until the butter melts and the sugar dissolves and the mixture is smooth, about 2–3 minutes. Remove from heat and stir in the vanilla extract, ground cinnamon, and ground nutmeg. Let the glaze cool for 1 minute.

Toss squash with oil and seasoning

Place the cubed squash in a large bowl. Drizzle the olive oil over the squash and sprinkle with kosher salt and black pepper. Toss to coat evenly. Pour about two-thirds of the warm brown sugar glaze over the squash and toss gently to coat each piece. Reserve the remaining glaze to brush on during roasting for extra caramelization.

Arrange and roast

Arrange the glazed squash cubes in a single layer on the prepared baking sheet, cut side up where applicable, leaving small gaps between pieces for air circulation. Roast in the preheated oven for 20 minutes.

Finish roasting with glaze

After 20 minutes, remove the baking sheet and gently flip the squash pieces with a spatula. Brush the tops with the reserved glaze. Return to the oven and roast an additional 10–15 minutes, until the squash is tender when pierced with a fork and edges are caramelized and lightly browned. Total roast time should be about 30–35 minutes depending on piece size.

Optional: add nuts

If using toasted pecans or walnuts, sprinkle them over the squash in the last 5 minutes of roasting so they warm and adhere to the glaze without burning. Alternatively, you can toss the warmed nuts with the roasted squash after it comes out of the oven.

Finish and serve

Remove the squash from the oven and let rest on the sheet for 3–5 minutes. Transfer to a serving dish, drizzle any accumulated pan juices or remaining glaze over the top, and sprinkle with the minced fresh parsley or thyme if using. Taste and adjust seasoning with a pinch of salt or pepper if desired. Serve warm.