Combine the flour, bicarbonate of soda and baking powder with ½ tsp salt and set aside.
In a separate bowl, beat the butter and sugars for 4-5 mins using an electric hand whisk until light and fluffy.
Add the egg and vanilla; whisk until combined.
Stir in the flour mixture using a wooden spoon until just combined.
Fold through 100g toffee pieces, then cover and chill for 30 mins.
Pour the sesame seeds onto a plate.
Using a tablespoon, scoop out spoonfuls of the dough and roll into 20 balls.
Roll each ball through the sesame seeds.
Sit the balls on lined baking sheets, leaving a 5cm space between each.
Press the remaining toffee pieces into the top of each ball and chill for 30 mins.
Meanwhile, preheat the oven to gas 5, 190°C, fan 170°C.
Bake the cookies for 10-12 mins until the edges are lightly golden.
Leave to cool completely on the trays, then carefully slide a knife underneath to release from the paper.
Store in an airtight container for 3-4 days at room temperature.
