1. In a medium saucepan (off the heat), whisk together the brown sugar and cornstarch until no lumps remain. This helps the cornstarch disperse evenly so your pudding thickens smoothly.
2. Slowly pour in about 1/2 cup of the milk while whisking to create a smooth paste. Once it’s lump-free, whisk in the remaining milk until fully combined.
3. Place the saucepan over medium heat. Cook, whisking constantly, until the mixture begins to steam and then thickens to a pudding-like consistency, 5–8 minutes. Don’t walk away—steady whisking keeps it silky and prevents scorching on the bottom.
4. Once the pudding has thickened and just starts to bubble, reduce the heat to low and cook for 1–2 more minutes, still whisking. This cooks out any starchy taste from the cornstarch.
5. Remove the saucepan from the heat. Stir in the butter (and the salt and vanilla, if using) until the butter is fully melted and the pudding is glossy and smooth.
