How to Make It
Grease a 9-by-13-inch baking pan. Whisk together flour, baking powder, baking soda, salt and 1 tsp. cinnamon in a medium bowl. In a small bowl, make topping: Mix together walnuts, nutmeg, 1/2 cup brown sugar and remaining 1/2 tsp. cinnamon.
In a large bowl, using an electric mixer on medium speed, beat together butter, sugar and remaining 1/2 cup brown sugar until light, about 3 minutes. Mix in eggs on low speed. Add 1/3 cup buttermilk, then 1/2 of flour mixture. Repeat until fully combined. Scrape batter evenly into pan. Sprinkle topping over, cover and refrigerate for 8 to 24 hours.
Preheat oven to 350°F. Uncover cake and bake until a toothpick inserted into center comes out clean, about 40 minutes. Let cool in pan on a wire rack for at least 10 minutes.