Preheat oven to 350°F.
For TOPPING: Whisk melted butter, corn syrup, brown sugar and lemon juice in a large microwave-safe bowl. Add apples; stir to coat. Microwave on HIGH (100%) for 1 to 3 minutes; stirring occasionally until apples just begin to soften. Pour mixture into a greased nonstick 9 x 2-inch round cake pan. Gently press apples into an even layer. Sprinkle almonds over apple mixture. Set aside.
For CAKE: Combine flour, baking soda and salt in a large bowl; set aside. Whisk sugar, brown sugar and egg in a large bowl until well blended. Gradually whisk in melted butter, then corn syrup, buttermilk and vanilla. Add flour mixture; whisk until just combined. Pour batter carefully over apples 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.
TIPS: Be sure to use a cake pan that has 2-inch sides; otherwise batter may overflow. For best results follow instructions for cooling cake. The 20 minute cool time before removing from pan is important for setting the topping.