Preheat oven to 350 degrees. Grease, flour and line 1 9" round baking pan with parchment paper. Set aside.
In the bowl of your stand mixer, whisk - using a regular whisk or the whisk attachment - the flour, sugar, baking powder and salt together very well. Change to the paddle attachment. Make a well in the center and add the softened butter, sour cream, egg, egg yolks and vanilla. Beat on medium speed until just combined and smooth. Do not over-mix! Fold in sprinkles by hand. Pour batter into prepared baking pan. Bake for 30-35 minutes. Check on the cake at 20 minutes and if you notice the top is browning too fast, cover the pan loosely with aluminum foil and bake until done. Cake is done when a toothpick inserted into the center comes out clean. Allow to cool in the pan for 20 minutes on a wire rack, then invert cake onto the wire rack out of the pan to finish cooling completely.
While cake is cooling, prepare the vanilla frosting. Beat the butter on medium speed with a hand or stand mixer until smooth with no lumps. Turn the mixer on low speed and add the confectioner's sugar, cream, vanilla and salt. Once the sugar is combined, turn to high speed and beat frosting until light and fluffy. If it seems too thick, add more cream and if it is too thin, add more confectioner's sugar. Once the cake is cooled completely, frost and decorate as desired. You may have some leftover frosting depending on how much you use.
Homemade Funfetti Cake With Fluffy Vanilla Frosting will stay fresh and moist for up to two days stored in an airtight container at room temperature.