Preheat the oven to 350°F(177°C). Line baking sheet with parchment paper or silicone baking mat. Set aside. (The dough is sticky, if you prefer, you can keep the dough in the fridge for 15 minutes to make easy cookie dough balls and preheat the oven when the dough is in the fridge.)
Place the flour, cornstarch, baking soda and salt if using in a medium bowl and stir with a whisk. Set aside.
Put the brown sugar, granulated sugar and butter in a large bowl and stir with a whisk.
Stir in the egg and mix until combined. Mix in the vanilla.
Add in the flour mixture and stir with a whisk. If you like you can continue with a spoon/spatula.
Add the oatmeal and combine with a spoon/spatula.
Stir in the white chocolate and dried cranberries evenly.
Roll the dough into 2 tablespoon-big (38 grams) balls, and place them on the baking sheet 2.5 inches (6.5 cm) apart from each other.
Bake for almost 10 minutes or until the cookies are slightly set at top and golden brown at the edges. These cookies will be too soft to handle when you remove them from the oven but they’ll be firm as they cool on the baking sheet for 20 minutes. All the cookies may not be all perfectly round. When you remove the cookies from the oven, give a round shape with a back of a spoon from the sides. Place on a wire rack to let them completely cool.