
Preheat oven to 350 degrees. Butter an 8-inch square cake pan. Mix together flour, baking soda and salt. In a separate bowl, stir together the granulated sugar, cinnamon and walnuts.
In a large bowl, beat the brown sugar and butter until fluffy. Beat in the eggs, 1 at a time, until fully combined, then beat in the vanilla and yogurt.Add the flour mixture in 2 batches, stirring until just combined.
Add the blueberries to the batter and gently combine. Spoon the batter into the pan, and then sprinkle the nut mixture over the cake. Bake until a knife inserted in the center comes out clean — about 35 to 40 minutes. Let cool and cut into squares.
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