Bring the milk to a simmer over low heat.

Whisk together the yolks, sugar and cornstarch in a large bowl. Slowly whisk the warm milk into the egg mixture, being careful not to scramble the eggs. Return the eggs to the pot over medium heat and bring to a boil, whisking constantly until thickened. This should not take long thanks to the cornstarch.

Scrape the mixture into another bowl and add the vanilla. Let cool to room temperature, cover with plastic wrap and refrigerate until cold, at least two hours.

Mix all ingredients in a small saucepan on medium/medium-high heat.

Let mixture cook, whisking gently for 9-10 minutes until the mixture thickens. Turn off heat and divide the mixture between two buttered, unlined 9” cake pans. Set aside.

Preheat the oven to 350F.

Cream together the butter and sugar until light and fluffy. Add the eggs and vanilla and beat well.

Mix together the dry ingredients in a separate bowl. Slowly add the dry to the egg/sugar mixture, alternating with the sour cream. Mix well.

Divide into the prepared, caramelized cake pans, careful not to disturb the caramel too much.

Bake 30-35 minutes until a cake tester comes out done. Let cool 15 minutes and then invert the cakes onto a cooling rack. The top should be all caramel-ly. Yum. Let cool the rest of the way.

In the large bowl of an electric stand mixer fitted with a whisk (if you have one), combine the eggs and yolks. Whisk on medium high speed for 5-8 minutes until the eggs become thick and lemon colored.

Meanwhile in a saucepan, combine the sugar and water. Heat over medium heat, stirring constantly with a rubber spatula until the sugar is completely dissolved. Be careful not to splash.

When the syrup looks clear, clip on a candy thermometer and increase the heat, bringing to a boil. Without stirring, let the syrup reach 238F, soft ball stage. Immediately remove the pan from the heat and pour the sugar into a glass measuring cup to stop the cooking.

As soon as the syrup is done cooking, slow the mixer and gently pour a small amount of syrup into the yolks and immediately increase the speed to high and beat about 10 seconds. Slow it down again, put in a larger amount of syrup and immediately turn the speed back to high. Repeat until all the syrup is gone. Work quickly since the sugar hardens quickly. Also, be careful to not add the syrup to the whisk because it will spin around the sides of the bowl.

Once the sugar has been added, beat the eggs about 5 minutes longer until the mixture is cool. The side of the bowl should feel cool to the touch.

If you have the paddle attachment, replace the whisk with that now. If not, just keep using the whisk. With the mixer on medium speed, add the butter, about 2 TB at a time, mixing 20-30 seconds after each addition (the eggs will deflate. Don't panic). Stop occasionally to scrape the sides. The buttercream may look curdled, but it will become smooth as you continue to beat it. Add the vanilla and beat until thick and smooth, about 5 minutes.

Now that the cake has cooled and the custard has set...

Slice each cake layer lengthwise. On a large plate, place one layer, caramel side down. Put about 1/3 of the custard filling. Repeat with all the cake layers. You should have four cake layers and three custard layers.