Melt butter in a small saucepan over medium heat; bring to a low simmer and cook, stirring frequently, until butter is browned, fragrant, and foamy, with brown bits in it, 5 -7 minutes. Set aside to cool to room temperature, about 15 minutes.
In a large bowl using a hand-held mixer or stand mixer with paddle attachment, beat the cooled brown butter, brown sugar, and granulated sugar together on medium speed until combined and creamy, about 2 minutes. Beat in the egg and vanilla. Scrape down the sides and bottom of the bowl as needed.
In a separate bowl, combine flour, cornstarch, baking soda and salt. Add dry ingredients into the wet ingredients, then beat on low speed until combined.
Add chocolate chips, diced guava, and coconut flakes to the bowl with the cookie dough and stir with a rubber spatula. Cover cookie dough with plastic wrap and chill for at least 1 hour and up to 2 days.
Preheat oven to 350°F. Line 2 large baking sheets with parchment paper or silicone baking mats.
Remove cookie dough from the refrigerator and allow to sit at room temperature for 10 minutes. Scoop cookie dough into 2 ounce balls and place on prepared pan. Bake for 10 minutes. Gently press down on the cookies with the back of a spoon and bake for an additional 5 minutes (15 minutes total). Remove from oven and cool for 5 minutes on the baking sheet. Transfer cookies to a cooling rack to cool completely. Enjoy!
