Preheat oven to 350º F. Cover bottom of a 12-inch skillet with parchment paper. In a medium bowl, combine graham cracker crumbs and butter until moist (I used a food processor to make crumbs). Evenly pat into bottom of skillet and bake 7 minutes; remove and let cool.
White Cake Batter
Beat together cake mix and butter on medium until combined. Add milk and eggs, beat on medium speed for 1 minute then medium-high for 2 minutes. Fold in toasted walnuts and white chocolate chips. Pour over crust, about half-way full; depending on depth of skillet you make not need all batter.
Brownie Cake Batter
In a large clean bowl, combine brownie mix, oil, water and egg; stirring with a fork or whisk for 2–3 minutes. Spoon large scoops over white cake batter to about 2/3 full, then use a butter knife to create desired swirl pattern, being careful to not over mix. If you have leftover batters bake in a separate dish.
Bake skillet at 350º for 25–30 minutes, or until top is set and white cake is just light golden color. Check center with a toothpick, you'll want it to come out moist with brownie batter to get fudge-like effect.
Depending on what brand you use for white cake and brownie mix, ingredients may change. Follow package direction for amounts and baking times.