In a heavy-bottomed saucepan, preferably nonstick, combine butter, the Redpath® Dark Brown Sugar, and lemon juice. Place over medium heat and cook while continuously stirring with a heat-proof spatula until butter and brown sugar is completely melted. Reduce heat to medium-low and continue to stir until mixture begins to darken in colour; about 3 minutes.
Pour in the condensed milk. Bring to a boil while continuously stirring mixture over medium-low heat to prevent mixture from separating (see Chef’s Tips). Cook for 1 to 2 minutes or just until mixture thickens and coats the back of a spoon. Do not overcook, or the toffee layer will become too hard to slice when completely cooled.
Remove from heat and stir in the vanilla extract and salt. Set aside to cool slightly, stirring every so often to prevent a skin from forming; 10 minutes.
Equally divide the warm toffee into the completely cooled pie crusts; about 3 to 4 tablespoons per tart. If necessary, use a small offset spatula to smooth the surfaces. Place the baking sheet with the filled tart shells into the fridge to chill while preparing the topping.