In a medium bowl, whisk together flour, baking soda, and salt until well combined.
In a large bowl, beat butter and brown sugar with an electric mixer until light and fluffy, about 3 minutes.
Add egg whites and vanilla extract; beat for 1 more minute.
Gradually add the flour mixture and mix until fully combined.
Fold in pecans and toffee bits until evenly distributed.
Cover the bowl and chill the dough in the refrigerator for at least 30 minutes.
Preheat oven to 375°F (190°C). Line two baking sheets with parchment paper.
Scoop heaping tablespoon-sized balls of dough and place them about 2 inches apart on the baking sheet. Flatten slightly with your palm.
Bake for 10–12 minutes, until edges are set and lightly browned.
Let cool on the baking sheet for 1 minute before transferring to a wire rack to cool completely.