Preheat the oven at 160ºC/320ºF conventional oven and line a cupcake tray with 12 cupcake liners.

Sift together the all-purpose flour, baking powder, baking soda and salt and set it aside.

In a mixing bowl, using the paddle attachment add the butter, and sugar and mix it on medium/high speed for 3 minutes.

Add the eggs one at a time into the mixture and mix on low until combined.

Scrape down the sides of the bowl and continue by adding half of the flour mixture, continuing on low speed, mixing until just combined.

Then add the sour cream, vegetable oil, vanilla extract and vanilla paste and mix until just combined.

Finally add the other half of the flour mixture and mix until just combined.

Using a rubber spatula, give the batter a gentle fold to make sure all of the ingredients have combined.

Divide the cake batter into the 12 cupcake liners and bake them for 20-22 minutes or until a cake tester comes out clean.

Let the cakes cool on a cooling rack. After 5 minutes, gently remove the cupcakes from the cupcake tin and let them cool down completely.

Sift the powdered sugar. Mix the butter in a stand mixer or with a hand mixer with the paddle attachment or with a hand mixer for 4 minutes on medium/high speed. Scrape down the sides of the bowl and mix for an additional 2 minutes.

Add the powdered sugar, vanilla paste and milk in 2 parts. Mixing on low speed until each addition is fully incorporated.

Scrape down the sides of the bowl and give the buttercream a final mix, on low speed, for 2 minutes.

Add the buttercream to a piping bag with a round piping tip e.g. a Wilton 1A.

Pipe the buttercream of the cupcakes, make a little indent in the top of the buttercream with the back of a teaspoon, and decorate with birthday sprinkles.