Preheat oven at 350 degrees and line 2 baking sheets with parchment paper.
In a medium bowl, whisk the flour, baking soda, cornstarch, and salt together then set aside.
In a large bowl, whisk the butter, oil, brown sugar, and granulated sugar together until no sugar lumps remain.
To the sugar mixture, add in the eggs and vanilla. Mix until combined.
Pour the dry ingredients into the bowl with wet ingredients. Gently mix until combined.
In a separate medium bowl, place your cubed guava paste and dust with about ¼ cup of flour. Make sure to coat all sides of the guava cubes in flour to make is easier to fold the cubes into your cookie dough.
Pour the cubed guava and flour into a strainer and sift out all excess flour.
Fold in the white chocolate and flour coated guava paste into the dough.
Using an ice cream scoop, scoop out the dough and place them on the prepared baking sheets. Leave about 2 inches of space between the cookies
Bake in the oven for 10-12 minutes.
Allow to cool before serving and enjoy!