Preheat oven to 350°F.
For Topping: Whisk melted butter, corn syrup, brown sugar and lemon juice in a large microwave-safe bowl. Add pears; toss gently to coat. Microwave on HIGH (100%) for 2 minutes; stir again. Pour mixture into a greased nonstick 9 x 2-inch round cake pan. Carefully use a fork to space pears evenly in pan with rounded sides down. Sprinkle walnuts over pear mixture. Set aside.
For Cake: Combine flour, ginger, cinnamon, baking soda and salt in a large bowl; set aside. Whisk sugar, brown sugar and egg in a large bowl until well blended. Gradually add melted butter, then corn syrup, buttermilk and vanilla, whisking until combined. Add flour mixture; whisk until just combined. Pour batter carefully over pears in prepared pan, smoothing top with a spatula if necessary.
Bake 35 to 40 minutes until cake is golden brown and toothpick inserted in center comes out clean. Cool pan 20 minutes on wire rack.
Run spatula or knife around edge of pan. Place a wire rack over cake pan. Invert cake onto wire rack; remove cake pan. (Place rack over baking sheet or large plate to catch any drips.) Cool 20 minutes; carefully transfer cake to serving platter. Cake is best served same day baked.
**NOTE: Bosc pears are best for this recipe. However, canned pears may be used instead. Use 1 can (16 ounces) pear halves, drained and halved. Follow recipe as directed in Step 2, but do NOT microwave pears and syrup. Continue with recipe as directed.
TIPS: Be sure to use a nonstick cake pan that has 2-inch sides; otherwise batter may overflow. Follow instructions for cooling cake. The 20 minute cool time before removing from pan is important for setting the topping.