Preheat oven to 350? F. Butter 13 x 9 x 2-inch glass baking dish. Using electric mixer, beat first 5 ingredients in large bowl on low speed until mixture resembles coarse meal. Transfer 3/4 cup butter-sugar mixture to medium bowl; mix in toffee and pecans.
Set aside. Stir baking soda into remaining butter-sugar mixture in large bowl. Add buttermilk, egg and vanilla, beating until just combined. Transfer batter to prepared dish.
Sprinkle toffee topping evenly over batter. Bake until topping is golden brown and toothpick inserted into center of cake comes out clean, about 35 minutes.
Cool completely in baking dish. Cut into 16 squares.
Store in airtight container at room temperature.