For Crumb Topping: Combine flour, sugars, graham pieces and salt in bowl of mixer fitted with paddle attachment. Mix in butter until combined. Set aside.
For Cake: Sift dry ingredients; set aside.
Cream butter and sugar in bowl of mixer fitted with paddle attachment on medium speed until light and fluffy. Add eggs, 1 at a time, mixing well after each addition. Stir in vanilla.
Gradually add dry ingredients alternately with sour cream, mixing on low speed, beginning and ending with dry ingredients. Scrape side of bowl as needed. Gently stir in graham pieces, being careful not to break them.
Place a piece of parchment paper on bottom of half sheet pan. Attach a 2-inch frame around edge of pan. Pour batter into prepared pan. Break up Crumb Topping and sprinkle evenly over batter.
Bake in 325°F standard oven 45 to 50 min. or until internal temperature registers 195-200°F. Cool completely in pan on wire rack. Carefully remove frame and cut 5 x 6 for 30 pieces.