With the rack in the middle position, preheat the oven to 375°F (190°C). Generously butter two 9-cup muffin tins.
In a bowl, combine the flour, baking soda and salt.
In another bowl, whisk together the melted butter and brown sugar. Add the egg yolk and vanilla. Whisk vigorously for 1 minute. With a spatula, fold in the flour mixture, then the chocolate chips. Place about 4 tsp of the batter in each muffin cup, pressing down lightly but without completely covering the bottoms. Set aside.
In a third bowl, melt the chocolate and butter in the microwave oven for 45 seconds. Stir until the chocolate has completely melted.
Add the brown sugar and egg. Whisk to combine. With a spatula, fold in the flour.
Place about 1 tbsp of the brownie batter over the cookie batter in each muffin cup.
Bake one muffin tin at a time for 8 minutes. Let cool on a wire rack for 15 minutes before unmoulding. Let cool completely on the wire rack for 2 hours.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.