- With the rack in the middle position, preheat the oven to 200°C (400°F). Butter a 43 x 30 cm (17 x 12-inch) baking dish and line it with parchment paper, letting the paper hang over two opposing sides.
- In a bowl, combine the flour and baking soda. Set aside.
- In a saucepan over low heat, melt the chocolate and butter with the cocoa powder. Whisk until smooth.
- In another bowl, beat the eggs and brown sugar until the mixture turns pale and triples in volume, about 5 minutes. With the mixer on low speed, add the dry ingredients, followed by the chocolate mixture.
- Spread the dough evenly on the pan. Bake until a toothpick inserted in the centre comes out clean, about 5 minutes. Let cool for 5 minutes. Unmould onto a cooling rack and remove the paper. Let cool completely.
- Cut the cookie wafer in half lengthwise.
- Cut half on the ice cream brick lengthwise into 3 equal rectangles 19 x 11.5 x 2 cm (7.5 x 4.5 x 3/4 inches). Return 1 of the rectangles to the freezer for another use.
- Arrange the two remaining ice cream rectangles, side by side, on the cookie sheet. Cover with the remaining cookie sheet. With a knife, trim away any excess ice cream.
- Cover with plastic wrap and freeze for about 30 minutes. Cut into 12 sandwiches. Wrap individually with plastic wrap and store in the freezer.
Kids will get an even bigger kick out of these bars if you decorate the sides with mini chocolate chips or coloured sprinkles. Spread some chips or sprinkles on a plate and gently press the sides of the bars into them.