Place frozen tart shells on rimmed baking sheet; place 1 tsp (5 mL) chocolate chips in each tart. Bake on lowest rack in 400°F (200°C) oven for 13 to 15 minutes or until golden and chocolate is melting; remove from oven. While hot, use back of spoon to spread chocolate over base and side of tarts; let cool.

Meanwhile, in medium bowl, combine egg yolk, flour and maple syrup until smooth; set aside. In small saucepan over medium heat, bring milk almost to boil; slowly and constantly whisk into egg mixture. Return to saucepan and cook over low heat until thickened, about 5 minutes. Transfer to clean bowl; place plastic wrap directly on surface of custard and refrigerate.

Meanwhile, in small saucepan over medium-high heat, combine rhubarb, maple syrup, liqueur and nutmeg; bring to boil. Reduce heat to low and simmer, stirring occasionally, for 10 minutes or until liquid is evaporated and rhubarb is soft. Let cool.

Place chocolate-lined tarts on platter; divide custard among shells (tarts can be assembled to this point up to 3 hours ahead). Just before serving, top with dollop of rhubarb compote; garnish with mint leaf.

Tip: Use any extra rhubarb compote on toast or over ice cream.