Preheat oven to 160°C fan-forced. Line a 12 capacity muffin pan with cupcake liners. Set aside.
In a large bowl, whisk together the flour and salt. Set aside.
In a separate bowl, beat the butter and sugar until light and fluffy. Beat in the eggs one at a time, then the vanilla. Add in sour cream and mix until fully combined.
Slowly add the dry to wet ingredients and mix until just combined. Scoop the batter into the cupcake liners. Bake for 18-22 mins or until a wooden toothpick inserted into the centre of the cupcake comes out clean.
Remove from the oven and allow to cool.
For the frosting: With a handheld mixer or stand mixer fitted with a whisk attachment, beat butter on high speed for about 3 minutes or until creamy and lightened in color, scraping down the bowl as needed.
Add icing sugar, cocoa powder, 3 Tbsp heavy cream, and 2 tsp vanilla extract.
Beat on low speed until sugar is incorporated then increase to medium/high speed and beat for 3 minutes, scraping down the bowl several times to ensure everything is well blended. Add another Tablespoon of heavy cream if needed to thin the frosting.
Transfer to a piping bag with and pipe small swirls on the top of cupcake.