Beat the softened butter until light and creamy.
Add the light brown sugar, caster sugar, and vanilla extract. Beat until smooth and fluffy.
Add the plain yogurt and mix until fully incorporated.
In a separate bowl, whisk together the all-purpose flour, cornstarch, baking soda, and salt.
Gradually add the dry ingredients to the wet ingredients and mix until just combined.
Fold in the chocolate chunks and roughly chopped roasted hazelnuts until evenly distributed throughout the dough.
Scoop equal portions of dough onto a parchment-lined baking sheet, leaving about 2 inches between each cookie.
Bake in a preheated 350°F (175°C) oven for 10–12 minutes, or until the edges are lightly golden and the centers are set.
Let the cookies cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely before serving.
