Preheat the oven to 325 degrees F. Line 2 large, heavy baking sheets with parchment paper.
Finely chop the oats in a food processor. Transfer the oats to a medium bowl. Mix in the flour, baking powder, baking soda, and salt. Set aside.
Using an electric mixer, beat the butter and sugars in a large bowl until fluffy. Beat in the eggs and vanilla. Add the flour mixture and stir just until blended. Stir in the toffee pieces, hazelnuts and chocolate chips.
Drop the dough onto the prepared baking sheets by rounded tablespoonfuls, spacing them 1 inch apart (do not flatten the dough). Bake until the cookies are golden (they will flatten slightly), about 15 minutes. Cool the cookies on the baking sheets for 5 minutes, then transfer them to a cooling rack and cool completely. (The cookies can be prepared 1 day ahead. Store airtight at room temperature.)