Preheat your oven to 350 degrees
In a large bowl, beat 1 cup of room temperature butter on medium speed until it is creamy. This can be done in a stand mixer or with an electric hand mixer
Add 1 cup of brown sugar, 1/2 cup of granulated sugar, 2 teaspoons of vanilla extract and 1 egg and mix until combined
Whisk together 2 1/4 cups of all purpose flour with 1 teaspoon of baking soda and 1/2 teaspoon of salt until fully combined
Add the flour mixture into the dough a little at a time until everything is fully incorporated. This dough is really stiff, but be sure to use all of the flour
Mix in the toffee bits, chocolate chips, m&ms, chocolate chunks, peanut butter chips and pretzels
Using a greased 1/4 cup measuring cup, drop large rounds of dough onto an ungreased non-stick baking sheet leaving 3-4 inches between each round of dough
Bake on a middle rack at 350 degrees for 12-14 minutes. The edges should be just beginning to brown, but the middles will still be a little soft
Transfer the baking sheets onto a cooling rack for 5 minutes and sprinkle the warm cookies with fleur de sel or sea salt
Transfer the cookies directly onto a cooling rack to cool and set the rest of the way