In a 10- to 12-inch nonstick frying pan over medium-high heat, tilt and stir sugar until melted and amber-colored, 8 to 10 minutes. Pour onto a sheet of foil (about 12 by 18 in.) and let cool to room temperature, about 20 minutes. Peel off foil and break caramel into chunks; put in a food processor or blender and whirl into a fine powder.
Combine milk and caramel powder in a 1 1/2- to 2-quart pan; stir over medium heat just until caramel is dissolved (it will form lumps, then melt; do not boil), 9 to 10 minutes.
In a small bowl, beat egg yolks to blend. Add cornstarch and whisk until smooth. Pour about 1/2 cup of the hot milk mixture into yolks and whisk until blended, then whisk yolk mixture into pan and stir over medium-high heat just until mixture boils and thickens, 3 to 4 minutes.
Rub pastry cream through a fine strainer set over a bowl; discard residue. Cover pastry cream and chill about 1 hour, or nest bowl in ice water and stir until cool, about 30 minutes.
In a large bowl, with a mixer on high speed, beat the whipping cream until it holds soft peaks. Gently stir the whipped cream into the cool pastry cream.