Preheat the oven to 350°F (175°C). Grease and line two 9-inch round cake pans with parchment paper.
In a medium bowl, whisk together cake flour, baking powder, baking soda, and salt.
In a large bowl, beat butter and sugar until light and fluffy. Add eggs and yolks one at a time, mixing well. Stir in vanilla extract.
Gradually add dry ingredients, alternating with sour cream and milk, mixing until just combined.
Divide the batter evenly between the cake pans and bake for 25-30 minutes, until a toothpick inserted in the center comes out clean. Let cool completely.
To make the pastry cream, heat milk and half of the sugar in a saucepan over medium heat until warm.
In a separate bowl, whisk egg yolks, remaining sugar, cornstarch, and salt. Slowly add the warm milk, whisking constantly.
Return the mixture to the saucepan and cook, stirring, until thickened. Remove from heat, stir in butter and vanilla extract, and cool completely.
Spread pastry cream over one cake layer, then place the second layer on top.
Serve as is, or dust with powdered sugar, add whipped cream, or garnish with fresh berries.
