1. Make the Donuts:

• Preheat your oven to 350°F (175°C) and grease a 12-cavity donut pan with nonstick spray.
• In a large mixing bowl, whisk together the dry ingredients: flour, baking powder, baking soda, salt, and nutmeg.
• In another bowl, whisk together the sugar, brown sugar, eggs, milk, sour cream, vanilla extract, and melted butter until smooth.
• Gradually add the wet ingredients to the dry ingredients, mixing until just combined. Be careful not to overmix; a few lumps are fine.
• Transfer the batter to a piping bag or a zip-top bag with the corner snipped off. Pipe the batter evenly into the prepared donut pan, filling each cavity about 3/4 full.
• Bake for 12-15 minutes, or until the donuts are lightly golden and a toothpick inserted into the center comes out clean.
• Let the donuts cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely.

2. Prepare the Custard Filling:

• In a medium saucepan, heat the milk and heavy cream over medium heat until it just begins to simmer. Remove from heat.
• In a separate bowl, whisk together the sugar, cornstarch, and egg yolks until smooth and pale.
• Gradually pour the hot milk mixture into the egg yolk mixture, whisking constantly to temper the eggs and prevent curdling.
• Return the mixture to the saucepan and cook over medium heat, whisking constantly, until it thickens and begins to bubble (about 2-3 minutes).
• Remove from heat and stir in the vanilla bean paste or vanilla extract.
• Strain the custard through a fine-mesh sieve to remove any lumps, then transfer it to a bowl. Cover the surface with plastic wrap to prevent a skin from forming and let it cool to room temperature. Refrigerate until ready to use.

3. Fill the Donuts:

• Once the donuts are completely cool, use a small knife or piping tip to cut a hole in the side or bottom of each donut.
• Transfer the custard to a piping bag fitted with a round tip. Insert the tip into each donut and fill with custard until it just begins to ooze out. Set the filled donuts aside on a wire rack.

4. Caramelize the Topping:

• Sprinkle about 1-2 teaspoons of granulated sugar evenly over the top of each filled donut.
• Use a kitchen torch to caramelize the sugar until it melts and forms a golden, glassy crust. Hold the torch about 2 inches from the surface and move it in small, circular motions to avoid burning.
• Allow the caramelized sugar to harden for 1-2 minutes before serving.

5. Serve and Enjoy:

• Serve the donuts immediately, or store them in an airtight container at room temperature for up to 1 day. Enjoy the contrast of the crunchy caramel topping, creamy custard filling, and fluffy donut base!