Begin by preheating your oven to 350°F (177°C). Line a mini muffin pan with 24 cupcake liners, ensuring they fit snugly.

In a large mixing bowl, combine the cake flour, baking powder, and salt. Whisk thoroughly to distribute the dry ingredients evenly.

In a separate bowl, mix together the melted butter and oil until they form a silky consistency. Gradually incorporate the granulated sugar while mixing on low speed until the mixture turns light and airy.

Add the room-temperature egg and vanilla extract, whisking until the mixture becomes smooth and cohesive.

Alternate between the dry ingredients, milk, and sour cream while mixing into the wet ingredients: start with one-third of the dry mixture, followed by half the milk, then another third of the dry mix, the sour cream, and finish with the remaining dry mix. Mix gently until just combined, being cautious not to overwork the batter.

Using a tablespoon-sized cookie scoop, fill each mini cupcake liner with batter, ensuring an even distribution.

Bake in the preheated oven for 10–14 minutes, or until the edges turn lightly golden and the tops spring back when gently pressed.

Allow the cupcakes to cool in the pan for 10–15 minutes. Then transfer them to a wire rack to cool completely before applying the frosting.

Using a stand or hand mixer, beat the room-temperature butter on low speed until smooth and creamy. Scrape down the bowl to ensure even consistency.

Add the sifted powdered sugar in small batches, about ½ cup (60 g) at a time, mixing each addition thoroughly before adding more. Continue until all the sugar is fully incorporated.

Mix in the vanilla extract, salt, and heavy cream. Beat the frosting on high speed until it achieves a fluffy texture and your desired consistency. Adjust the cream quantity for a softer or firmer frosting as needed.

Frost each cooled mini cupcake with 1–2 tablespoons (15–30 ml) of buttercream. Decorate as desired and savor every bite!