Preheat the oven and chop the pistachios: Preheat the oven to 350°F (175°C). Line your cookie baking sheet with parchment paper or a silpat. Rough chop the pistachio, reserve and set aside ¼ cup of the chopped pistachios for sprinkling later.

Melt the butter: Place the butter in a large bowl and melt in the microwave or use a saucepan to melt the butter on the stovetop and transfer the melted butter to a large bowl. Set aside.

Combine dry ingredients: In a medium bowl, whisk (affiliate link) together the flour, baking soda, cornstarch, and salt. Set aside.

Combine the butter and sugars: Add the brown sugar, and granulated sugar to the butter and whisk (affiliate link) until there are no lumps. Add the egg and vanilla extract to the butter mixture and whisk (affiliate link) until well combined.Gradually add the flour mixture to the wet ingredients and mix just until combined. 

Fold in pistachios: Gently fold in the rough chopped pistachios.

Scoop and roll cookie dough: Measure about 2 ounces of cookie dough using a cookie scoop or tablespoon measuring spoon. Roll into a ball and place on the cookie sheet spacing them about 2 inches apart.

Bake cookies: Bake for 7 minutes, until the edges are golden but the centers are still soft. They will firm up as they cool. Allow cookies to cool for 5 minutes, then transfer to a wire rack to cool completely.

Melt the chocolate: While the cookies are cooling, melt the chocolate using the double boiler method or in the microwave.

Stovetop double boiler method: Fill a small pot with about 1-2 inches of water and bring it to a gentle simmer over medium heat. Place a heatproof bowl on top of the pot, making sure it doesn't touch the water. Add the semi-sweet chocolate chips. Stir the chocolate continuously as it melts, ensuring it melts evenly and doesn’t overheat. Once smooth and fully melted, remove from heat. Tip: Be careful not to let any water or steam get into the chocolate, as this can cause it to seize and become lumpy.

Microwave method: Place the semi-sweet chocolate in a microwave-safe bowl. Heat the chocolate in 20-30 second intervals on medium power. After each interval, stir the chocolate thoroughly to help distribute heat and avoid burning. Continue microwaving and stirring until the chocolate is smooth and completely melted. Tip: Avoid microwaving for too long at once, as chocolate can easily burn.

Dip or spread the chocolate: Once the cookies are cool, use an off-set spatula or back of a spoon to spread half of each cookie with the melted chocolate and place them on the cooling rack placed over parchment paper to catch any drips. Sprinkle some of the reserved chopped pistachios on the chocolate side for extra crunch and decoration.

Set the chocolate: Let the chocolate harden at room temperature, or place the cookies in the refrigerator for faster setting.