Lightly spray a 9 x 13 pan with cooking spray and line with parchment paper. Preheat the oven to 350°F.
In a medium bowl, whisk together the flour, baking soda, salt and nutmeg.
In a large bowl, cream the butter and sugars until light and fluffy. Beat in the eggs and yolk until fully incorporated. Then, mix in the extracts.
Stir half of the flour mixture into the butter mixture on low speed until just combined. Repeat with the remaining flour. Fold in the chocolate chunks.
Spread the dough evenly into the prepared pan. Bake for 35 minutes, until the top is golden brown all over.
Let the bars cool completely in the pan, for about 2 hours. Then lift them out of the pan using the edge of the parchment paper. Cut into 12 bars using a serrated knife.
Store the bars in an airtight container. These are best if eaten within a few days, but they also freeze well. Enjoy!