In a large bowl, whisk together flour, baking, soda and salt and set aside
Grease muffin tins or line with cupcake liners
In the bowl of a stand mixer, cream butter and sugar until fluffy (2-3 minutes)
Add eggs, sour cream and vanilla and beat well - mix until the egg yolks are completely combined and batter is a cream color
Stir flour mixture into cake batter mixture mixing until fully blended
Spoon cake batter into each muffin tin, filling about 3/4 to the top
Bake at 350 degrees for 12-14 minutes or until a toothpick comes out clean from the center
Remove from oven and allow to cool for 30 minutes
BUTTERCREAM
Add flour, salt and milk to a small saucepan
Cook over low heat until thick, stirring constantly. As soon as the mixture thickens, remove from heat. Transfer to a small bowl and allow to cool. This step is very important.
In the bowl of a stand mixer, cream butter and sugar together well until light and fluffy, then add vanilla
Add the flour mixture and beat on medium speed until fluffy (about 3-4 minutes)
Using a piping bag, insert the tip about 3/4 of the way into each muffins and pipe buttercream until it fills to the top of each muffins, then add a dollop of cream to the top of each muffin.