Preheat and prepare the pan

• Preheat the oven to 175 °C.
• Line an 8x8 or 9x9 baking pan with parchment paper.

Mix the wet ingredients

• In a bowl, combine the melted butter with the sugar and brown sugar and mix until smooth.
• Add the egg, egg yolk and vanilla extract and mix well until fully combined.

Add the dry ingredients and chocolate chips

• Stir in the flour, baking soda and salt until just combined; do not overmix.
• Fold in the chocolate chips.

Layer the dough and Nutella

• Press half of the dough evenly into the prepared pan.
• Spread the Nutella evenly over the dough.
• Cover with the remaining dough, gently pressing and sealing the edges, then sprinkle extra chocolate chips on top if desired.

Bake

• Bake until the edges are set and the center is slightly soft.

Cool and slice

• Let the bars cool completely in the pan before slicing into squares.