Freeze the Nutella balls

• Scoop tablespoon-size balls of Nutella onto a lined plate or tray.
• Freeze until firm.

Beat butter and sugars

• Beat the softened butter, brown sugar and granulated sugar until creamy.

Add egg and vanilla

• Mix in the egg and vanilla extract until combined.

Add dry ingredients

• Add the flour, cornstarch, baking soda, baking powder and salt.
• Mix until just combined.

Fold in the chocolate

• Fold in the mini semi-sweet chocolate chips, milk chocolate chips and M&M's.

Chill the dough

• Place the dough in the freezer.

Stuff the cookies

• Scoop the dough and flatten it slightly.
• Place a frozen Nutella ball in the center and completely wrap the dough around it, making sure the Nutella is fully sealed inside.

Freeze the stuffed dough balls

• Place the stuffed cookie dough balls in the freezer.

Bake

• Bake at 190 °C (375 °F) just until the edges are set and lightly golden; the centers should still look soft and slightly underbaked.

Decorate and round the edges

• While the cookies are still warm, press extra M&M's into the tops.
• Use a cup or cookie cutter to gently swirl around each cookie to round out the edges.

Cool

• Let cool on the baking sheet.