Preheat oven to 350°F (175°C). Grease and flour a 9×13-inch baking dish.,In a medium bowl, whisk together flour, baking soda, salt, cinnamon, and nutmeg.,In a large bowl, cream butter and granulated sugar until light and fluffy. Add egg and vanilla; mix until combined.,Alternate adding the dry ingredients and buttermilk to the creamed mixture. Mix until just combined.,Spread half the batter into the prepared pan.,In a small bowl, mix together brown sugar, cinnamon, and nuts if using. Sprinkle half over the batter.,Spread the remaining batter on top. Sprinkle with remaining cinnamon-sugar mixture and drizzle with melted butter.,Bake for 30–35 minutes, or until a toothpick inserted in the center comes out clean.,Cool slightly before slicing and serving.