Spray an 8x8-inch baking pan with nonstick spray. (This helps the parchment or foil stick to the pan.)

Line the pan with parchment paper or foil, allowing 2 edges to extend over the sides. Then line again in the opposite direction so the entire pan is covered. This makes removing the fudge from the pan easy.

Attach a candy thermometer to a medium-large pot, positioning the lower end about ½ inch from the bottom. Remove and set aside — this will make it easier to reattach when the pot is hot later.

Add the brown sugar, granulated sugar, half and half, pumpkin purée, corn syrup, pumpkin spice, and salt. Stir to combine.

Bring to a boil over medium heat, stirring frequently. Continue boiling and stirring for 5 minutes.

Attach the candy thermometer and cook, stirring occasionally, until the temperature reaches 234°F (soft-ball stage) — this will take about 5–8 minutes.

Once the temperature hits 230°F, watch closely — the temperature can climb quickly. Remove the pot from the heat immediately at 234°F.

Stir in the white chocolate melts until fully melted and smooth.

Add the marshmallow crème and vanilla extract; stir vigorously until fully combined.

Holding the pot about 12 inches above the pan, pour the fudge mixture into the prepared pan. (Pouring from above helps reduce air bubbles.)

Tap the pan on the counter a few times to smooth the surface.

Sprinkle lightly with flaky sea salt, if desired.

Cool on a wire rack until completely set, then refrigerate for 30 minutes before cutting.

Lift the fudge from the pan using the parchment, peel away the liner, and cut into 1-inch squares. Serve at room temperature.