An excellant treat for those cold winter mornings or packing for lunches...just warm and  add a pad of butter..

Ingredients

• 1/2 cup packed brown sugar
• 1/4 cup chopped walnuts or pecans
  
• 2 teaspoons ground cinnamon
• 1 cup granulated sugar
• 1/4 cup butter, softened
• 2 large egg whites 
• 1 cup reduced-fat sour cream 
• 1 teaspoon vanilla extract
• 1 3/4 cups all-purpose or whole wheat flour
• 1 teaspoon baking powder
• 1 teaspoon baking soda
• 1/2 teaspoon salt
• Cooking spray 

Preparation

1. Preheat oven to 350°.
2. Combine first 3 ingredients; set aside.
3. Place granulated sugar and butter in a large bowl; beat with a mixer at medium speed until well-blended (about 5 minutes). Add egg whites, 1 at a time, beating well after each addition. Beat in sour cream and vanilla. Lightly spoon flour into dry measuring cups; level with a knife. Combine flour, baking powder, baking soda, and salt, stirring well with a whisk. Gradually add flour mixture to sugar mixture; beat well. Spread half of batter into an 8-inch square baking pan coated with cooking spray. Sprinkle half of walnut streusel over batter. Spread remaining batter over streusel. Top with remaining streusel...Do the same but in greased muffin tins
4. Bake at 350° for 45 minutes or until a wooden pick inserted in center comes out clean. Cool the cake on a wire rack

Source:http://www.myrecipes.com/recipe/sour-cream-coffee-cake-10000000226250/