Preheat oven to 350°F. Spread the walnuts in a single layer on a baking sheet. Toast for 6-8 minutes until golden. Set aside to cool.
Spray a 13 x 9 inch baking pan with cooking spray.
Using an electric mixer, cream together butter, brown sugar, egg and vanilla.
In a separate bowl, use a whisk to sift together flour and salt. Add to the creamed butter mixture beating on medium just until fully combined. Mix in ½ toffee bits by hand.
Spread into the prepared baking pan. Bake for 25 minutes until golden.
Remove from the oven and immediately sprinkle the mini chocolate chips on top. Allow to sit on the counter for 5 minutes until melted, then spread evenly.
Immediately sprinkle with toasted walnuts, remaining chocolate covered toffee bits and sea salt.
Cool completely then cut into squares. Store in an airtight container at room temperature.