How to Make It
In a medium bowl, whisk the flour with the baking powder, baking soda and salt.
In a large bowl, using a handheld electric mixer, beat the butter and brown sugar at medium speed until creamy, about 4 minutes. Beat in the egg, and then the yolk and vanilla extract until light and fluffy. Reduce the speed to low and add the dry ingredients, mixing until just combined. With a spatula, fold in the mini chocolate chips and toffee pieces until evenly distributed. Cover the mixing bowl and refrigerate the dough for at least 3 hours or overnight for the best results.
Preheat the oven to 375° with the racks placed in the lower and upper-middle positions. Line two baking sheets with parchment paper and set aside.
Scoop the dough into 1/3 cup-size balls, and arrange at least 3 inches apart on the prepared baking sheets. Bake for about 12 to 14 minutes, rotating the pans halfway through, until the cookies are golden brown but still soft and slightly under baked in the center. Let the cookies cool on the baking sheets for 2 minutes before transferring them to wire racks.