Preheat oven to 350°F. Grease 9" round cast iron pan.
Spread walnuts in an even layer on baking sheet. Toast in oven for 8-10 minutes until fragrant. Set aside to cool.
25 g chopped walnuts
Whisk together flour, baking powder, baking soda, and salt in large bowl.
180 g all-purpose flour, 1/2 teaspoon baking powder, 1/4 teaspoon baking soda, 1/2 teaspoon salt
In the bowl of a stand mixer, or with a hand mixer, cream together butter and both sugars until light and fluffy. About 5 minutes. Scrape down sides of bowl as needed.
57 g unsalted butter, 80 g dark brown sugar, packed, 25 g granulated sugar
Add oil, egg, and vanilla, mixing thoroughly after each addition.
2 tablespoons canola or vegetable oil, 1 large egg, 1 teaspoon vanilla extract
Slowly add in flour mixture. Mix in chocolates and walnuts. Mix with rubber spatula by hand to ensure no streaks of flour remain.
50 g dark chocolate chips, 50 g milk chocolate chips
Spread dough in an even layer in cast iron pan. Bake for 18-20 minutes until edges are brown and center is set but still soft.
Serve warm with ice cream or allow to cool completely before serving.