Make the Lemon Pastry Cream

In a medium saucepan, heat the milk over medium-low heat until steaming.

In a separate bowl, whisk together egg yolks, sugar, lemon zest, salt, vanilla, and cornstarch until smooth and pale yellow.

Slowly pour in 1/4 of the hot milk while whisking vigorously. Then gradually whisk in the rest.

Return the mixture to the saucepan and cook over medium-low heat, whisking continuously for 8–12 minutes, until it thickens and soft peaks form.

Remove from heat and whisk in the butter until smooth.

Transfer to a bowl, press plastic wrap directly onto the surface, and refrigerate until fully chilled.

Make the Lemon Sugar

In a small bowl, mix sugar and lemon zest. Set aside for coating cookies.

Prepare the Cookie Dough

Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper.

In a bowl, whisk together flour, baking powder, baking soda, and salt.

In another bowl, use an electric mixer to cream the butter and sugar for 2 minutes until fluffy.

Add the egg, vanilla, and lemon zest. Beat on medium speed until light and pale, about 1–2 minutes.

Gradually add the dry ingredients and mix on low just until a soft dough forms.

Shape and Bake

Scoop dough using a large cookie scoop. Roll into balls.

Roll each dough ball in the lemon sugar.

Place on baking sheet and slightly flatten each. Bake 6 at a time.

Bake for 9–10 minutes, until edges are set and centers are puffy.

Let cookies cool on the pan for 5 minutes, then transfer to a wire rack to finish cooling.

Assemble the Cookies

Transfer chilled pastry cream into a piping bag fitted with a small round tip.

Pipe a swirl of pastry cream onto each cooled cookie.

Sprinkle about 1 tsp of granulated sugar over each cookie top.

Use a kitchen torch to brûlée the sugar until golden and caramelized.

Let cookies cool for 10 minutes before serving to allow the topping to harden.