In medium heatproof bowl, whisk egg yolks, granulated sugar, 2 tbsp (25 mL) of the maple syrup and salt until smooth and pale yellow. Gradually whisk in cornstarch until smooth and well combined. 

In medium saucepan, over medium heat, bring milk and cream to boil. Gradually whisk into yolk mixture. Return mixture to pan. Bring to gentle boil, whisking continuously. Simmer, stirring, for 1 to 2 minutes or until slightly thickened. Remove from heat; stir in remaining maple syrup and vanilla. Transfer custard to clean medium bowl. Place plastic wrap directly on surface. Refrigerate until cold, about 3 hours. (Can be refrigerated for up to 3 days.) 

Line baking sheet with parchment paper. On lightly floured work surface, roll puff pastry out to about 1/4 inch (5 mm) thickness. Trim to two 5- x 10-inch (12 x 25 cm) rectangles; place on baking sheet, spacing apart. Cover loosely; refrigerate for 20 minutes until chilled. 

Prick pastry several times with fork. Spread chilled custard evenly onto each pastry, leaving 1/2-inch (1 cm) border. Peel, core and slice apples into thin wedges. Starting at one end of pastry, lay apple slices on custard, overlapping slightly. 

Bake in 400°F (200°C) oven 15 to 20 minutes, until pastry is puffed and pale golden-brown. Sprinkle with brown sugar. Return to oven and bake for another 4 to 5 minutes, or until sugar is caramelized. 

Remove tarts from oven; transfer to wire rack and cool. Cut each into 3 pieces. Serve warm or at room temperature. Sprinkle with icing sugar, if desired.