Preheat oven to 350°F (175°C) and grease a 9x13-inch baking dish. In a large bowl, combine yellow cake mix, eggs, milk (or buttermilk), melted butter, and vanilla. Blend until smooth.

In a separate bowl, mix together brown sugar, ground cinnamon, and chopped nuts until evenly combined. Set aside for topping.

Pour half of the cake batter into the greased baking dish. Sprinkle half of the topping mixture over the batter. Layer remaining cake batter and top with remaining mixture.

Bake for 45-50 minutes until edges are golden brown and center is set but slightly moist. Check doneness using a toothpick.

Remove from oven and let cool for 10-15 minutes before cutting into squares and serving warm or at room temperature.