Heat the oven to 350°.
Line an 8-inch square pan with a foil (or parchment paper) sling: In the pan, crisscross two foil pieces so that each edge extends 1 or 2 inches beyond the lip of the pan. Smooth the foil onto the pan edges and into the corners. Grease with butter.
In a small bowl, whisk together the flour and salt.
In a separate bowl, using a mixer on high, beat the butter and sugar until light and fluffy, about 3 minutes. Beat in the egg and vanilla until the ingredients are combined, scraping down the bowl as needed. With the mixer on low, gradually add the flour mixture and blend until a sticky dough forms.
With the back of a spoon or the heel of your hand, press the dough evenly into the prepared pan. Bake until the crust is golden brown, about 15 to 20 minutes.
In a small bowl, stir together the pecans and toffee bits.
Remove the pan from the oven and immediately sprinkle the chocolate chips over the crust. Let them sit for a few minutes to melt, then use an offset spatula or the back of a spoon to spread the melted chocolate evenly. Sprinkle the top with the pecan and toffee mixture.
Set the pan on a wire rack to cool completely, about an hour. If the chocolate is still not set, cool the pan in the refrigerator for 20 to 30 minutes. Use the foil sling to lift the bars from the pan. Cut them into squares.