Toffee Coffee Cake

1/2 cup butter or margarine, softened
1 cup firmly packed brown sugar
1/2 cup granulated sugar
2 cups all-purpose flour
1 cup buttermilk
1 egg
1 teaspoon baking soda
1 teaspoon vanilla extract
3 (1.4 ounce) chocolate English toffee candy  bars
    (such as Heath