Preheat oven to 350°F (175°C).

Grease and line an 8-inch cake pan.

In a large bowl, cream softened butter and sugar with an electric mixer until light and fluffy.

Add eggs one at a time, beating well after each addition.

Beat in vanilla extract.

Sift flours together.

Fold flour mixture into the creamed mixture alternately with sour cream and lemon zest until well combined.

Fold in thawed blueberries and chopped walnuts.

Spread batter into prepared cake pan.

Bake for 35-40 minutes, or until a toothpick inserted into the center comes out clean.

Cool cake in pan for 5 minutes before turning out onto a wire rack.

Brush the top of the cake with melted butter.

Sprinkle cinnamon sugar over the buttered top while hot.

Cool completely before serving.