Mix the dry ingredients

• In a bowl, combine the all-purpose flour, baking soda, baking powder, milk powder, espresso powder, cornstarch and salt.
• Mix well to combine.

Cream the butter and sugars

• In a stand mixer fitted with the paddle attachment, add the cold cubed butter, brown sugar and granulated sugar.
• Mix on medium speed until the mixture looks creamy.

Add the egg and vanilla

• Add the egg, egg yolk and vanilla bean paste.
• Mix until fully combined, scraping down the sides of the bowl as needed.

Combine wet and dry

• Gradually add the dry ingredients to the wet mixture.
• Mix on low speed just until combined, without overmixing.

Fold in the chocolate chips

• Stir in the semi-sweet and milk chocolate chips until evenly distributed.

Chill the dough

• Cover and refrigerate the dough for at least 2 hours, or up to 24 hours for deeper flavor and thicker cookies.

Preheat the oven

• Preheat the oven to 175 °C.
• Line a baking sheet with parchment paper.

Shape the cookies

• Scoop large portions of dough and roll loosely into balls.
• For a bakery look, keep them tall rather than flattened.

Bake

• Bake until the edges are set and lightly golden, while the centers stay soft and slightly underbaked.

Finish and cool

• Sprinkle flaky sea salt on top immediately after baking.
• Let the cookies cool on the baking sheet before transferring to a wire rack.