Preheat oven to 450°F (232°C). Butter 12 muffin tins; set aside.
Unroll pastry and remove parchment paper. Re-roll pastry sheet tightly into cylinder. Cut into 12 equal pieces. Gently roll out each piece to about 4 inches (10 cm) to fit into buttered muffin tins; refrigerate until ready to use.
In a saucepan, whisk together sugar and flour until combined. Whisk in cream, egg yolks and salt until smooth. Place saucepan over medium low heat and cook, whisking for about 10 minutes or until very thick and smooth. Remove from heat and scrape into a bowl. Whisk in vanilla. Let cool to room temperature, whisking often.
Fill each pastry cup with custard and bake in upper third of oven for about 10 minutes or until pastry is golden brown. Let cool about 15 minutes before eating.