To prepare oatmeal crumb: Combine butter, 1/2 cup brown sugar, 1/2 cup oats, 1/4 cup flour and 1/2 teaspoon cardamom (or cinnamon) in a food processor. Process until the mixture is crumbly. Turn out into a bowl and add the remaining 1/2 cup oats and walnuts. Combine with fingertips or a fork until blended.

To prepare cake: Preheat oven to 350°F Coat an 8-inch-square pan with cooking spray.

Sift 2 cups flour, baking powder, 1 teaspoon cardamom (or cinnamon), baking soda and salt together in a large bowl. Whisk eggs and 1/2 cup brown sugar in a medium bowl until well blended, gradually whisk in buttermilk, oil and vanilla. Add the wet ingredients to the dry ingredients in 2 additions, stirring each time to thoroughly blend the ingredients together.

Spread half the batter in the prepared pan. Sprinkle half the oatmeal crumb evenly on top. Spoon the remaining batter over the crumbs and gently spread in an even layer. Top with the remaining oatmeal crumb.

Bake the coffee cake until browned and a toothpick inserted in the center comes out clean, 35 to 40 minutes. Let cool for 10 minutes. Serve warm.