In a medium saucepan, whisk together granulated sugar, cornstarch, and flour until combined. Gradually add milk and cream, stirring continuously until well mixed. Stir in egg yolks and vanilla seeds. Cook over medium-low heat, whisking constantly until the mixture thickens and bubbles emerge, about 7–10 minutes. Once thick, transfer to a bowl, cover with plastic wrap, and refrigerate for at least 4 hours, allowing flavors to meld.

Preheat your oven to 350°F (175°C) and line a muffin tin with cupcake liners. In a large bowl, whisk together the dry ingredients: flour, cornstarch, baking powder, and salt. In another bowl, cream softened butter, granulated sugar, and vanilla extract until fluffy. Add eggs one by one, mixing well after each addition. Gradually alternate adding the dry mixture and milk combined with oil until just incorporated to create a smooth batter.

Fill the muffin cups two-thirds full with the cupcake batter, ensuring an even distribution. Bake in the preheated oven for 18–20 minutes, or until a toothpick inserted into the center comes out clean and the tops are lightly golden. Let the cupcakes cool in the pan for 5 minutes, then carefully transfer them to a wire rack to cool completely, ensuring they are not soggy.

Once the cupcakes are completely cooled, use a small knife or cupcake corer to cut out a cone shape from the center of each cupcake. Discard the tops or save them to create a "hat" for the filled cupcakes. Carefully spoon or pipe chilled pastry cream into the hollowed centers of each cupcake, allowing the cream to slightly overflow. Top each filled cupcake with a dollop of additional pastry cream and refrigerate to set.

When ready to serve, sprinkle a generous layer of coarse sugar onto the tops of each cupcake. Using a kitchen torch, carefully melt and caramelize the sugar until it forms a golden, crisp crust. Serve immediately after torching for the best experience of the crunchy sugar shell atop your delightful Crème Brûlée Cupcakes!