Preheat your oven to 350 degrees Fahrenheit (175 degrees Celcius). This ensures that the bars cook evenly. It’s important to give your oven enough time to reach the right temperature.
Line a 9x9-inch baking pan with parchment paper. This will help prevent the bars from sticking, making it much easier to remove them once they’re baked. For an extra precaution, lightly spray the parchment with non-stick spray.
In a medium saucepan over medium heat, melt the unsalted butter. Keep an eye on it, stirring occasionally to ensure it doesn’t brown too much. This melted butter will be the base of your cookie batter.
When the butter is melted, remove it from the heat and let it cool for a few minutes. Once it’s cooler, scrape the butter into a large heat-proof mixing bowl.
Add the light brown sugar and granulated sugar to the bowl. Whisk them together until they are well combined. The mixture should look smooth and velvety.
Stir in the Baileys Irish Cream and vanilla extract. Make sure these are fully incorporated into the sugar mixture. This is where the bars get their unique flavor!
Add in the whole eggs and the additional egg yolk. Beat until everything comes together nicely. The batter will begin to thicken.
Using a rubber spatula, fold in the all-purpose flour, baking soda, ground cinnamon, salt, and espresso powder. Be careful not to over-mix; you want to combine the ingredients just until you see no more dry flour.
Next, gently fold in the semi-sweet chocolate chips. This is the moment when you can really get excited, as those chocolate chips will create delightful pockets of chocolate in the bars.
Scrape the batter into the prepared pan. Use your spatula to spread it evenly. The batter might be thick, so take your time to ensure it’s evenly distributed.
Bake in the oven for 30 to 34 minutes. Look for the edges to be firm and golden while the center should still be slightly wobbly. This will create that soft, chewy texture.
Once baked, remove from the oven and cool in the pan for at least 1 hour before cutting. If you sprinkle with flaky sea salt, do it while the bars are still warm.
