1. Make the Custard

1. Heat the milk and heavy cream until steaming (do not boil).
2. In another bowl, whisk egg yolks, sugar, and cornstarch until smooth.
3. Slowly whisk the hot milk mixture into the egg mixture.
4. Return everything to the saucepan and cook over medium heat, stirring constantly, until thick.
5. Stir in butter and vanilla.
6. Cover the surface with plastic wrap and chill completely.

2. Prepare the Cookie Dough

1. Preheat oven to 350°F (175°C).
2. Line two baking trays with parchment paper.
3. Whisk together flour, baking powder, baking soda, and salt.
4. Beat butter and sugar until light and fluffy.
5. Mix in egg, vanilla, and sour cream.
6. Gradually add the dry ingredients until a soft dough forms.

3. Bake

1. Scoop about 2 tablespoons of dough for each cookie.
2. Roll into balls and place on the baking trays.
3. Slightly flatten each cookie.
4. Bake for 10–12 minutes, until the edges are lightly golden while the centers remain soft.
5. Cool completely.

4. Fill the Cookies

1. Spoon or pipe chilled custard onto the center of each cooled cookie.
2. Smooth the top with the back of a spoon or a small offset spatula.

5. Torch the Sugar

1. Sprinkle about 1 teaspoon of granulated sugar evenly over each custard-filled cookie.
2. Use a kitchen torch to caramelize the sugar until deep amber and crisp.
3. Let the topping cool for 1–2 minutes before serving.