Preheat the oven to 350°F. Line an 8 inch square baking pan and line with parchment paper, let it hang over both sides so it is easier to take out once done baking. Coat with cooking spray.
In a stand mixer with whisk attachment, whisk butter, and brown sugar until smooth, about 2 minutes. Add in the eggs, one at a time and vanilla extract until combined.
Remove bowl from mixer and stir in flour and fleur de sel until just combined. Do not overmix. Fold in 1 cup of chocolate chips, ½ cup crushed pretzels and peanuts.
Pour batter into prepared baking pan, smoothing out the top and sprinkling with remaining peanuts and whole pretzels.
Bake for 35-40 minutes until a toothpick inserted comes out clean. While cookies are baking, prepare chocolate sauce.
Melt the ½ cup of chocolate chips remaining with the vegetable oil in a double boiler or microwave.
Remove the cookie bars from the oven, let them cool completely before removing from the pan with the parchment paper. Drizzle the bars with the prepared chocolate sauce and allow it to cool. Cut the bars into 16 squares and serve.