Preheat oven to 375 degrees F and line a baking sheet with a silicone mat.
Whisk together flour, cocoa powder, cornstarch, baking soda, and salt in the bowl of your stand mixer. Set aside.
In a separate bowl, melt butter in microwave.
Once the butter has melted, whisk in both sugars until fully incorporated.
Stir in the egg and vanilla.
Mix the wet mixture into the dry mixture until combined.
Mix in the white chocolate chips, pecans, cashews, and semi-sweet chocolate chips.
Bake for 8 minutes. Or until the tops of the cookies have set.
Remove from oven and let cool on baking sheet for 2 minutes.
After the 2 minutes, remove from the baking sheet and let cool the rest of the way on a cooling rack!
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