Combine the milk, cream, and half of the sugar in a medium sauce pan over medium-high heat, stirring occasionally until the liquid comes to a simmer.

As the liquid heats, whisk together the remaining sugar, salt, cornstarch, and egg yolks in a medium bowl until light and creamy.  

Once the milk mixture is hot, slowly whisk about 1 cup of the liquid into the egg mixture to temper the yolks. Pour the tempered egg mixture back into the saucepan with the hot milk mixture and reduce the heat to medium, continuing to cook while whisking constantly, until thickened and a few bubbled burst on the surface, about 30 seconds.  

Remove the pastry cream from the heat and whisk in the butter, vanilla, and almond extract, then transfer to a bowl and cover with a sheet of plastic wrap placed directly onto the surface of the pastry cream so a skin doesn't form. Refrigerate until chilled and firm, about 3-4 hours, or overnight.

Preheat oven to 350 degrees F. Line muffin pans with cupcake liners. Whisk the cake flour, baking powder, baking soda, and salt together in a large bowl.

Beat the butter on high speed in a stand mixer or using a handheld mixer for 1 minute until creamy and light. Add sugar and beat another 2 minutes. Scrape sides and bottom of the bowl.

Add eggs and vanilla, beating on medium-high speed until combined. Beat in sour cream.

Add the dry ingredients, mixing on low speed just until combined, then scrape the bottom and sides of the bowl.

With the mixer running on low speed, add the milk, stirring just until combined. Do not overmix.

Fill each cupcake liner about ⅔ full with batter, then bake for 19-22 minutes, until a toothpick inserted in the center of a cupcake comes out clean. Cool completely before filling and frosting.

Combine the egg whites and sugar in the bowl of a stand mixer set over a pan of simmering water.  Be careful not to let the bowl touch the water.  Whisk until sugar is dissolved and the egg whites are hot (between 140-160 degrees F is enough to kill any bacteria in the egg whites and dissolve the sugar). 

Transfer the bowl on the stand mixer and whip the sugar and egg whites on high using the whisk attachment until thick and glossy, forming stiff peak and the bowl feels room temperature to the touch, which can take anywhere from 10-20 minutes.  The stiff peaks will form fairly quickly, but just let the mixer keep working to help bring the temperature of the meringue down.  This can be done with a hand mixer, but it's much easier with a stand version.

Once the bowl is no longer warm to the touch, decrease the speed to medium-low and add the butter, 1 tablespoon at a time, beating until smooth after each addition.  Once all of the butter has been beaten in to the meringue, mix in the vanilla and almond extracts and salt. 

If the frosting looks curdled or clumpy, increase the mixer speed to medium-high and beat until smooth.  Or if the frosting is too runny and thin, stick it in the fridge for 10 minutes to help it set up a bit, then whip it again.  This is a problem I've encountered because I didn't let my meringue cool down enough before adding the butter, but the fridge fix worked great.

Preheat the oven to 350 degrees F.  Cover a baking sheet with foil and spray it with nonstick cooking spray.  Pour the nuts onto the baking sheet in an even layer and toast them in the oven while you prepare the caramel.  Be careful to watch them so they don't actually burn.  They should just start to turn a light, toasty brown and smell nutty.

Combine the water, sugar, lemon juice, and salt in a small saucepan over medium-high heat, stirring just until the sugar dissolves.  Insert a candy thermometer and let the mixture boil without stirring until it reaches 310 degrees F, then remove from the heat and stir in the butter and toasted almonds immediately.  

Pour the caramelized nuts back onto the foil-lined baking sheet and use a spatula to spread them into as thin a layer as possible, trying not to have too many overlapping nuts.  Cool completely, then break apart, chopping large chunks if necessary.

Use a small sharp knife to cut a plug out of each cupcake. Fill with pastry cream, then replace the plug on top. You will have pastry cream leftover which is great for cream puffs, eclairs, small tarts, or just folding into some whipped cream and using as a crepe filling!

Pipe a thick layer of swiss meringue buttercream on top of each cupcake. It doesn't have to look pretty, but you want to completely cover the plug to hold it in place. Again, you will have leftover swiss meringue buttercream, but I find it so much easier to make a larger batch of this than to halve the batch. The leftover frosting freezes great though for up to 3 months in an airtight container! Just thaw in the fridge overnight, then beat until light and fluffy to use on other desserts.

Roll and press the tops of each cupcake in a bowl of the candied almonds to completely cover the frosting, then serve!