In a large bowl, whisk together the oats, all-purpose flour, cake flour, cornstarch, baking powder, baking soda, cinnamon, and salt. Set aside.

Using a handheld mixer or a stand mixer fitted with a paddle attachment, beat the butter, brown sugar, and granulated sugar together on medium-high speed until creamy and combined, about 3 minutes. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) Add the eggs, molasses, and vanilla, and beat on high speed until combined, about 1 minute. Scrape down the sides and bottom of the bowl and beat again as needed.

Add the flour mixture to the wet ingredients, plus the chopped chocolate, dried cranberries, and walnuts. Beat on low speed until combined. This is a thick, heavy, and sticky dough, so you may need to start and stop your mixer a couple times.

Cover and refrigerate the dough for at least 1 hour (and up to 4 days). If chilling longer than 1 day, let the dough sit at room temperature for at least 30 minutes before rolling, as it will be very firm.

Line large baking sheets with parchment paper or silicone baking mats. Set aside.

Use a 1/3-cup measuring cup to scoop the cookie dough, about 3 ounces (85g) each, and arrange 5–6 cookies on each baking sheet.

Bake for 5 minutes; then, without opening the oven, reduce the temperature to 350°F (177°C) and bake for an additional 10 minutes.

Remove the cookies from the oven and allow them to cool on the baking sheet for 10–20 minutes before transferring to a wire rack to cool completely.

Cookies stay fresh covered at room temperature for up to 1 week.