Preheat oven to 350 ° F. Grease 2-6-inch round cake pans and line with parchment paper. Set aside.
In the bowl of a stand mixer, cream together butter and sugar. Add eggs and yolk, one at time. Add sour cream and vanilla paste. Mix until combined.
In another bowl, whisk together flour, baking powder, baking soda, and salt. Add dry mixture to mixing bowl in three additions, alternating with heavy cream. Begin and end with the dry ingredients. Mix until just combined. Be careful not to overmix and incorporate too much air into the batter.
Weigh batter and divide evenly between two cake rounds. Smooth the top, making sure batter is settled nice and even. Bake for 20-30 minutes until cake is done and toothpick inserted in the center of cake comes out clean. Let cool in pan for five minutes before unmolding and allowing to cool completely on a cooling rack. Cool to room temperature before slicing layers.