Preheat oven to 350 degrees. Line bottom and sides of an 8-inch square baking pan with foil, with a bit of overhang on two sides. Set aside.
In a mixing bowl, cream butter and sugar until smooth. Beat in eggs, vanilla and salt. With mixer on low speed, add flour and beat until combined. Stir in toffee bits by hand.
Spread batter in prepared pan. Bake cake until golden and a toothpick inserted in center comes out clean, 30 to 35 minutes. Remove from oven; cool completely in pan.
Grasping foil, lift cake from pan. Peel off foil. Using serrated knife, cut into 16 squares. Store blondies in an airtight container at room temperature up to 5 days.