Preheat oven to 350°.

Spray a 9-inch springform pan with nonstick cooking spray.

In a small bowl, combine raisins, walnuts, 1/4 cup brown sugar, and cinnamon; set aside.

In a large bowl, beat butter, sugar, and remaining 1/4 cup brown sugar at medium speed with a mixer until creamy.

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

Beat in vanilla.

In a medium bowl, combine flour, baking powder, baking soda, and salt.

Gradually add to butter mixture, beating just until combined.

Stir in sour cream.

Spoon half the batter into prepared pan.

Top evenly with reserved raisin mixture.

Spread remaining batter evenly over raisin mixture.

Bake for 45-55 minutes or until a pick comes out clean.

Remove sides of pan.

In a small bowl, combine powdered sugar and cream.

Drizzle over cake.

Garnish with walnuts, if desired.

Cut into wedges and serve.