Line an 8x8-inch or 9x9-inch baking pan with parchment paper or foil, leaving an overhang on two sides for easy lifting. Lightly spray with nonstick spray. For thinner fudge squares, you can use a 9x13-inch pan.

In a medium skillet over medium heat, melt 1 tablespoon unsalted butter. Add the pecan halves or pieces and cook, stirring often, for 4–6 minutes, until fragrant and a shade darker. Watch closely so they don’t burn.

Transfer the toasted pecans to a plate to cool slightly. If you started with pecan halves, roughly chop them once cooled. Set aside, reserving a small handful for topping if desired.

In a large, heavy-bottomed saucepan (at least 3 quarts), add 1 cup (2 sticks) unsalted butter and melt over medium heat. Continue cooking for 2–3 minutes, stirring, until it smells nutty and has just a hint of golden color. Do not let it get too dark.

Stir in the light brown sugar, granulated sugar, heavy cream, and fine sea salt. Cook over medium heat, stirring until the sugars dissolve and the mixture is well combined.

Bring the mixture to a gentle, steady boil over medium heat, stirring often. Once it is bubbling across the surface, cook for 4–5 minutes, stirring so it doesn’t scorch. The mixture should thicken slightly and look glossy. If using a candy thermometer, cook to 234–238°F (soft-ball stage).

Remove the pan from the heat and place it on a heat-safe surface. Immediately add the marshmallow creme and vanilla extract. Stir steadily until completely smooth and fully combined.

Gradually stir in the sifted powdered sugar, about 1/2 cup at a time, mixing well after each addition. The fudge will change from glossy to thick and matte and should become very thick but still spreadable, similar to a very thick cake batter or soft frosting.

Fold the toasted, cooled pecans into the fudge mixture, reserving a small handful for topping if desired. Taste and, if you like a more pronounced sweet-salty contrast, add a small pinch of extra salt and stir again.

Quickly pour the fudge into the prepared pan and spread it evenly with a spatula. Sprinkle the reserved pecans evenly over the top and gently press them in so they adhere. Let the fudge cool at room temperature for 2–3 hours, or until firm enough to cut. You can move it to the refrigerator after about 45 minutes to speed things up, but avoid the freezer at this stage.

Use the parchment overhang to lift the set fudge out of the pan. Place on a cutting board and use a sharp knife to cut into 1- to 1 1/2-inch squares, wiping the knife with a warm, damp cloth between cuts for clean edges.