- 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, 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.