Ingredients

• 1 cup (250 mL) butter, softened
• 2-1/2 cups (625 mL) packed brown sugar
• 4 eggs
• 1 tbsp (15 mL) vanilla
• 2-1/2 cups (625 mL) all-purpose flour
• 1-1/4 tsp (6 mL) salt
• 1-1/4 cups (300 mL) semisweet chocolate chips
• 2 cups (500 mL) marshmallows

• Preparation

  
  
  In large bowl, beat butter with sugar until fluffy; beat in eggs, 1 at a time. Beat in vanilla. Whisk flour with salt; stir into butter mixture in 2 additions. Stir in 1 cup of the chocolate chips just until combined. Spread in parchment paper–lined or greased 13- x 9-inch (3 L) baking dish; smooth top.
  
  Bake in 350ºF (180ºC) oven until cake tester inserted in centre comes out with a few moist crumbs clinging, about 25 minutes.
  
  Sprinkle with remaining chocolate chips. Sprinkle with marshmallows, leaving 1/2-inch (1 cm) border to prevent sticking to sides of pan. Bake until marshmallows are puffed, about 3 minutes. Let cool in pan on rack. (Make-ahead: Cover and refrigerate for up to 3 days or freeze for up to 2 weeks.) With greased sharp knife, cut into squares.