Preheat the oven to 325°F and line the muffin pan with cupcake/muffin liners.
Infuse the sugar with vanilla. In a medium-sized bowl add sugar and vanilla beans or paste (if using) and pure vanilla extract. (see note 1)
Beat the eggs until pale in colour. Add the room temperature eggs into the vanilla sugar and beat until pale in colour and sugar is almost dissolved.
Ingredients to make the cupcakes moist. Add the oil and sour cream and mix until well blended.
Measure, mix and sift the flour, baking powder and salt. Add to the egg mixture in four additions. Mix in just until incorporated.
Heat the milk and melt the butter over the stove or microwave for a few seconds until it reaches 120°F to 125°F. (see note 2)
Mixing in the hot liquid. Add the hot milk-butter mixture slowly while mixing into the cake batter. Mix until blended. (see note 2)
Use a liquid measuring cup to fill the muffin pan/cups with the liner about a quarter cup or 2/3 full.
Bake the vanilla cupcakes. Bake the cupcakes in preheated oven at 325°F for about 15 to18 minutes until springy to the touch or when the inserted toothpick comes out clean.
Cooling the cupcakes. Remove the vanilla cupcakes from the oven and let them cool in the muffin pan for about 5 minutes. Then remove from the muffin pan and transfer to a cooling rack to cool completely.