Preheat the oven to 350℉/180℃ and line two large cookie sheets with parchment paper.
In the bowl of a stand mixer fitted with the paddle attachment (or with a handheld mixer) cream together the butter and brown sugar on medium high speed for 5 minutes.
½ cup unsalted butter, 1 cup light brown sugar
Scrape down the sides, then add the eggs and vanilla extract and beat on medium speed for an additional minute.
2 large eggs, ½ tablespoon vanilla paste or extract
Whisk together the flour, baking soda and salt, then add it to the wet ingredients. Mix on low until it starts to combine, then scrape down the sides and finish mixing until no flour streaks remain.
¾ teaspoon baking soda, 2½ cups all-purpose flour, ½ teaspoon kosher salt
Add the chopped candy and mix on low for a few turns, just until it's evenly dispersed. You can also do this by hand to be sure the pieces don't break up.
1½ cups leftover halloween candy
Use a large cookie scoop to scoop the dough (fill the scoop completely, then scrape it on the side so it's totally flush), then add the dough to the pan. Only add 4-5 at a time, keeping them spread evenly.
Press extra pieces of candy into the tops of the dough, then bake for 10-12 minutes until the edges are set and starting to brown. The centers will still look underdone.
Remove the pan from the oven and smack it on the counter a few times. Then, use a round object with an opening larger than the cookies to scoot the cookies to reshape them. Add additional pieces of chocolate candy to the tops for extra melty bits, then sprinkle each cookie liberally with flaky sea salt.
flaky sea salt
Repeat with the additional batches of cookies.
Cool the cookies completely on the pan before transferring to a wire rack or storage container.