Beat butter, brown sugar, and granulated sugar in the bowl of a stand mixer with the paddle attachment at medium speed until light and fluffy, about 3 to 4 minutes. Add the vanilla and then add the eggs, one at a time. Beat well after each addition. Scrape the bottom and the sides of the bowl as needed with a rubber spatula.
In a large bowl, whisk together the flour, baking soda, cornstarch, cinnamon, and salt. Stir in the oats. With the mixer on low speed, gradually add the dry ingredients to the butter and sugar mixture, beating until a dough is formed. Using a spoon or spatula, stir in the walnuts, cranberries, and white chocolate chips. Cover with plastic wrap, and refrigerate for 15 to 30 minutes until it firms up.
Preheat oven to 375°F (190°C). Line 2 baking sheets with parchment paper.
Using 2-tablespoon spring-loaded cookie scoop, scoop the cookie dough, and place 2 inches apart on the prepared pans. If the dough is very cold and firm, flatten cookie balls slightly with your hand or the bottom of a glass.
Bake for 10 to 12 minutes. Let cool on pans for 3 to 4 minutes. Remove from pans, and let cool completely on wire racks.
Store the cookies in an airtight container for up to 2 weeks. (Place a small sheet of parchment or wax paper between each cookie to prevent sticking.) Or freeze cookies individually wrapped in plastic wrap in an airtight freezer-safe container for 2 to 3 months.