Preheat oven to 350°. In a large bowl, cream 1/2 cup butter with the granulated sugar until light and fluffy. Beat in eggs and vanilla.
In a medium bowl, sift together flour, baking powder, baking soda, and salt. Stir into butter mixture. Add yogurt and mix; gently stir in blueberries.
In a small bowl, stir together the brown sugar, cinnamon, nutmeg, and walnuts. Pour half the cake batter into a buttered and floured 8-inch square baking pan. Sprinkle with half the brown sugar mixture and top with the remaining batter.
Combine the remaining brown sugar mixture with 2 tablespoons butter and sprinkle over the top of the batter. Bake until toothpick inserted into center comes out clean, about 45 minutes.
Note: Nutritional analysis is per serving.