Split the vanilla beans in half and remove the seeds. Place the vanilla bean seeds and milk in a saucepan over medium heat along with the vanilla bean pods and bring to almost a simmer. If using vanilla extract instead of vanilla bean seeds, you should add it later once the custard is done cooking, so for now, just heat up the milk by itself on the stove.

Once you see the first bubbles emerging on the milk, turn the heat off.

Whisk egg yolks, sugar, and cornstarch in a bowl for a couple of minutes, until pale in color.

Pour about 1/4 cup of hot vanilla milk over the yolks while whisking. Do this slowly so you don’t cook the eggs. This process is called tempering the eggs, and you are supposed to add the milk slowly in order to not cook the eggs.

Add another little bit and whisk. Do this until the milk has all been incorporated.

Pour the custard back into the pan that you used to heat up the milk, through a strainer to contain any bits of egg that might have cooked, and to retain the vanilla bean pods.

Place the pan over medium heat, and begin to cook the custard stirring non-stop.

Cook the custard for a few minutes until it becomes thick.

When the custard starts thickening up, the mixture will look scrambled or lumpy. Just continue to stir and cook, it will come together and become smooth.

When the custard is thick like pudding, remove from the heat.

Place the custard in a bowl. If you didn’t add vanilla bean seeds, now it’s the time to add the vanilla extract to it. Stir to combine.

Place plastic wrap directly on the surface of the custard so it won’t form a skin as it cools down.

Let it cool down, and then place it in the fridge for at least 2 hours.

You can make the pastry cream a day ahead.

Pre-heat the oven to 350ºF.

Line a 8” or 9” springform cake pan with parchment paper and spray it with oil. If using a 8" pan, make sure it's deep.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set it aside.

In a large mixing bowl, or in the bowl of a stand mixer, beat the butter at medium speed for 30 seconds.

Add the sugar and the vanilla bean seeds to the bowl, and beat together with the butter for another 2 minutes, until the mixture is light and fluffy.

Scrape the bowl every so often to ensure everything is getting nicely incorporated together.

Add the eggs, one at a time, mixing until incorporated before adding the next egg.

Add the vanilla and mix to combine.

Add the sour cream and mix until incorporated.

Add half of the dry ingredient mixture to the bowl and mix on low to combine.

Pour the buttermilk in and mix.

Add the remaining dry ingredient mixture and mix on low to combine. You can also use the spatula instead of the mixer to fold the batter during this final stage. Avoid over mixing, to prevent the cake from turning out dry.

Spread the batter on the bottom of the prepared pan.

Baking

You will know the cake is done baking when you touch the top of the cake and doesn’t feel soft or jiggly underneath. Insert a toothpick in the cake, and should come out clean, without any gooey batter attached.

Remove the cake from the oven and let it cool down completely.

When the cake has cooled down, use a dowel, or a chopstick, or even the handle of a wooden spoon or spatula, to poke holes in the cake.

Place half of the pastry cream in a piping bag. Snip the end of the bag with scissors.

Pipe the pastry cream inside each hole you poked.

Top the cake with the remaining pastry cream.

Use a spatula to spread it out evenly and smooth it out on top of the cake.

Place the cake in the fridge for at least 30 minutes before serving.

Sprinkle a thick coat of sugar on top of the cake. The sugar will melt a bit into the pastry cream, just sprinkle more on top until there’s a thin white coating of sugar on top of the cake. This can take from 1/4 cup of sugar to 1/3 cup.

Use a torch to toast the sugar.

Wait a few minutes for the sugar to cool down slightly, and as it cools down it will harden and become crispy.

Slice and serve the cake.