In a the bowl of an electric stand mixer, fitted with whisk attachment whisk butter and sugar on medium-high speed until very pale and fluffy or about five minutes.
Turn mixer OFF and add in cocoa powder, vanilla and salt.
Turn mixer on low and blend for about 30 seconds.
One tablespoon at a time add in heavy cream. Keep scraping down the bowl until all the cream is incorporated.
Now jack up the mixer to medium-high to high and whisk for 3-5 minutes or until mixture becomes beautifully light and oh so fluffy.
Recipe by SIMPLY BEAUTIFUL EATING at https://simplybeautifuleating.com/2018/11/13/my-kids-favourite-marble-bundt-cake/