Preheat the oven to 350°F and line 3 baking sheets with parchment paper.
Put all of the mini Heath Bars in a Ziploc bag. Crush them with a rolling pin until you have a mix of big and little pieces. Set aside.
In a large mixing bowl, cream together the butter, sugar, and brown sugar until very light and fluffy, about 5 minutes.
Beat in the eggs one at a time, then mix in the vanilla. Scrape down the sides of the bowl.
Add 2 cups of flour and mix just until combined. Then, in a small bowl, dissolve the baking soda in the hot water. Add the baking soda mixture, the remaining 1 cup of flour, the salt, and cream of tartar to the bowl. Mix just until combined.
Add the crushed Heath Bars and mix on low or fold by hand until everything is evenly distributed.
Using an ice cream or cookie scoop, portion the dough onto the parchment-lined baking sheets. Leave enough space between each cookie.
Bake for 10 minutes or until the edges look set and lightly golden. Depending on the size of your oven, you may have to do this in batches. Let the cookies cool for approximately 30 minutes. The Heath pieces should re-crisp as the cookies cool, so you get that classic crunch.