1
: Preheat oven to 350°F (180°C). Line baking sheets with parchment paper.
2
: Filling: Melt chocolate with sweetened condensed milk in a small heavy saucepan over medium heat. Reduce heat to low and stir until mixture thickens, about 5 minutes.
Remove from heat, stir in vanilla. Cool in refrigerator.
3
: Beat butter, sweetened condensed milk and brown sugar until well combined. Add egg and vanilla; beat well. Add flour and baking powder. Beat ingredients just until combined. Mix in all the chocolates. Cover and refrigerate dough for 15 minutes.
4
: Roll 1 tbsp (15 mL) dough into balls. Place on prepared baking sheets, about 2” (5 cm) apart. Flatten slightly.
5
: Bake in preheated oven 10-12 minutes or until lightly golden. Cool on wire rack.
6
: Place 1 tsp (5 mL) cooled filling on the underside of a cookie. Top with a second cookie, underside down. Repeat until all cookies are sandwiched.