Place a saucepan over medium-high heat and add the milk and vanilla extract. Bring to a gentle simmer, then remove from heat. Scoop out ? cup of the hot milk and set it aside.

In a small mixing bowl, whisk together the sugar and cornstarch until well combined.

Add the egg yolks and whisk until smooth, then slowly pour in the reserved ? cup of milk while whisking constantly.

Return the saucepan to the stovetop over low heat and slowly pour the egg yolk mixture into the warm milk, whisking constantly.

Continue whisking until the mixture thickens, then remove from heat and add the butter. Mix until the butter melts and the mixture is silky. Transfer the pastry cream to a bowl, press plastic wrap directly onto the surface, and refrigerate until fully cooled and set.

In a large mixing bowl, add the softened butter, granulated sugar, brown sugar, vanilla extract, and kosher salt. Beat with an electric hand mixer until creamy and well combined.

Add the egg and egg yolk, and mix until smooth.

Add the flour and baking soda, and mix just until the dough comes together.

Use a large cookie scoop and roll into smooth balls. Place the dough balls on a parchment-lined baking sheet.

Bake in a preheated oven at 180?C (350?F) for 9?11 minutes, until the edges are lightly golden. Let the cookies cool on the tray for 5 minutes, then transfer them to a wire rack to cool completely.

Once cooled, transfer the pastry cream to a piping bag fitted with a round tip and pipe a generous swirl onto each cookie.

Sprinkle with granulated or caster sugar, then use a kitchen torch to caramelize the sugar until it melts and turns golden brown.