Cream together softened butter and granulated sugar until light and fluffy, about 2-3 minutes. Add egg yolks and vanilla bean paste, mixing until smooth.

In a separate bowl, whisk together flour, cornstarch, baking powder, and salt. Gradually add to butter mixture, mixing just until combined.

Wrap dough tightly in plastic wrap and refrigerate for at least 30 minutes. Preheat oven to 350°F (175°C).

Use a cookie scoop to portion dough onto parchment-lined baking sheets, about 2 inches apart. Bake for 10-12 minutes until edges are golden.

After cooling for 5 minutes, sprinkle caster sugar on top of each cookie and caramelize with a kitchen torch.

Allow cookies to cool on a wire rack before serving.