Preheat oven to 375 degrees. On a lightly floured surface, roll out dough 1/8 inch thick. Using a 5-inch cutter, cut 12 rounds, re-rolling the dough as needed. Brush off excess flour, and gently fit rounds into 3 1/2-inch tart pans with removable bottoms. Trim edges with scissors, a paring knife, or a rolling pin. Place tart pans on a baking sheet. Chill for 30 minutes.
Using a slotted spoon, remove peaches from poaching liquid. Reserve poaching liquid for another use. Arrange three peach wedges in each tart shell.
In a medium bowl, whisk together the egg and sugar until thick and pale yellow. Add the flour and salt, and whisk until smooth. Whisk in the cream and cognac. Spoon custard around peaches in tart shells. Sprinkle lightly with sugar. Bake until the custard is puffed and golden, about 35 minutes. Transfer to wire racks to cool. Serve at room temperature.