Preheat oven to 350 degrees and line a baking sheet with parchment paper or a Silpat.
In a medium bowl, whisk together flour, baking soda, and salt. Set aside.
In a stand mixer or in a bowl with hand mixer, cream together butter, granulated sugar, and brown sugar until light and fluffy, 3-4 minutes.
Mix in egg and vanilla until just combined.
With the mixer on low, blend in flour mixture until just combined, taking care not to over-mix.
Fold in 3 cup popped popcorn and toffee pieces.
Scoop about 3 Tbsp cookie dough (I use a #20 cookie scoop) onto prepared cookie sheets, about an inch apart.
Bake for 10-12 minutes, until cookies are just turning golden around edges. Let cool in pan for a couple minutes before moving to wire racks to cool completely.