Cream the softened butter, dark brown sugar, and granulated sugar in a stand mixer on medium-high speed for 3-4 minutes until light and fluffy.

Add the egg and vanilla extract to the creamed mixture, mixing on low speed for about 30 seconds until smooth.

Gradually incorporate the baking powder, baking soda, salt, and flour, mixing on low until just combined.

Fold in the oats and toffee bits until evenly distributed.

Shape the dough into balls, pressing a soft caramel into the center before sealing and placing on a parchment-lined baking sheet.

Chill the dough balls in the refrigerator for at least 2 hours.

Preheat the oven to 365°F (185°C) and prepare another baking sheet with parchment paper.

Bake the chilled dough for 10-12 minutes, until golden brown around the edges but soft in the center.

Sprinkle each warm cookie with flaked salt and let cool before transferring to a wire rack.