Preheat the oven to 350 F (177 C). Line a 24 ct. mini muffin pan with mini cupcake liners.
In a mixing bowl, whisk the cake flour, baking powder, and salt together until well combined.
1 cup (120 g) cake flour, 1 teaspoon baking powder, ¼ teaspoon salt
Whisk the melted butter and oil together until smooth and well combined. With the mixer on low, stream in the sugar. Mix the sugar with the oil and butter mixture until light and fluffy.
4 tablespoon (57 g) unsalted butter, melted, 2 tablespoon (30 ml) oil, ¾ cup (150 g) granulated sugar
Next, whisk in the egg and vanilla extract until smooth and well combined.
1 large egg (US), room temperature, 1 teaspoon (5 ml) vanilla extract
Alternating with the dry, whisk in the sour cream and milk into the wet in this order: dry, milk, dry sour cream, dry until just combined. Do not overmix.
¼ cup (59 ml) milk, room temperature, ¼ cup (60 g) sour cream, room temperature
Use a tablespoon sized cookie scoop to scoop the mini vanilla muffin batter into each of the mini cupcake liners.
Bake the muffins for 10 - 14 minutes or until the sides of the mini cupcakes are lightly golden and the tops bounce back quickly when lightly pressed. Cool the cupcakes in the pan for 10-15 minutes. Gently remove them from the pan to cool completely before frosting them. Cream the room temperature butter on low until smooth. Stop and scrape down sides of bowl as needed. Mix in the sifted powdered sugar ½ cup (60 g) at a time, beating each addition of powdered sugar until incorporated. Scrape down sides of mixing bowl as needed.
½ cup (113 g) unsalted butter, room temperature, 2½ cups (300 g) powdered sugar, sifted
Once all the sugar has been added, mix in the vanilla extract, salt and heavy cream. Beat on high until the vanilla buttercream is fully incorporated and your desired consistency.
1 teaspoon (5 ml) vanilla extract, ? teaspoon salt, 2 - 3 tablespoon (30 - 45 ml) heavy cream
Top each of the cooled mini cupcakes with 1 - 2 tablespoons (15 - 30 ml) of frosting and enjoy!