In a medium saucepan, whisk together sugar, cornstarch, and salt. Whisk in milk, and place over medium heat.

Continue whisking and cook until thickened and bubbling. Whisking all the while, cook for 2 additional minutes. Remove from heat.

In a medium bowl, whisk egg. While whisking egg rapidly, very gradually pour in half of the hot liquid. Pour egg custard back into saucepan.

Return to medium heat, and cook about 2-3 minutes. Remove from heat.

Add butter and vanilla, and stir until butter is completely melted.

Pour into bowl, lay plastic wrap on the surface to prevent a skin from forming, and refrigerate for at least 2 hours.

To serve, scoop into bowls.