Scoop 1 tablespoon pistachio cream onto a parchment-lined plate 10 times. Freeze for 20–30 minutes until firm. This helps keep the filling centered during baking.

In a saucepan over medium heat, melt the butter. Cook, stirring frequently, until the butter foams and the milk solids turn golden brown and smell nutty. Pour into a large mixing bowl and immediately stir in the milk powder and espresso powder. Let the brown butter sit for at least 10 minutes.

Add the brown sugar and granulated sugar to the cooled brown butter. Whisk until smooth and glossy.

Whisk in the egg, egg yolk, and vanilla extract until fully combined.

Add the flour, cornstarch, baking soda, baking powder, and salt. Fold until a soft dough forms and no dry streaks remain.

Scoop large portions of dough (about 3–4 tablespoons each). Flatten slightly and place a frozen pistachio cream portion in the center. Wrap the dough around the filling and roll into a ball. Refrigerate the filled dough balls for at least 1 hour, or up to 48 hours.

Bake at 350°F (175°C) for 11–13 minutes, until the edges are lightly golden and the centers look slightly underbaked. These are large bakery-style cookies and will continue setting as they cool.

Let cookies cool for 10–15 minutes. While still slightly warm, optionally dollop a little extra pistachio cream on top so it gently melts over the cookie. Sprinkle with coarse sugar, sanding sugar, or cane sugar if desired.