Heat oven to 350°F. Whisk melted butter and sour cream together in a large bowl. If the butter was still warm from melting, this should cool it. Whisk in egg, salt, and sugar until smooth. Add flour and combine with a spoon until a crumbly, loose mass forms. Transfer dough to your counter and knead a few times, just until smooth. Divide dough into 4 equal pieces. The total dough weighs about 630 grams; each quarter will weigh about 157 grams.

Roll first quarter of dough between two pieces of parchment paper until it's in a very thin 8″x10″ rectangle. If yours is a little wider or shorter, that's fine; you'll just want the remaining pieces to be the same size so they stack neatly. Peel away top parchment sheet and set aside to use for next dough. With the dough still on the bottom parchment sheet, use a knife or pastry wheel to cut dough in half, into two 4″x10″ rectangles. No need to separate them. Dock the dough all over with a fork and slide parchment and dough onto a baking sheet big enough that it lays flat. Bake for 9 to 12 minutes, or until light brown at edges. Transfer wafers to cooling rack.

Repeat with remaining quarters of dough. If you'd like to use less parchment, you can wait until the first quarter is baked and cooling to reuse the parchment for the remaining quarters. If you'd like to use less time and have the oven space, use additional sheets of parchment to roll out the remaining quarters and bake more than one at a time. Wafers can be stacked as they cool.

In a medium saucepan, whisk together sugar, starch, and salt. Add the eggs, one at a time, whisking until smooth and no pockets of sugar-starch remain before adding the next. Whisk in vanilla bean paste, if using, and then, very gradually, whisking the whole time, pour in milk. Bring mixture to a simmer over medium heat, whisking the whole time. As the custard begins to bubble, it will thicken. Simmer for one minute, whisking. Remove from heat and stir in the butter until it is fully melted, then the rum (if using) and vanilla extract. If you want your custard extra silky-smooth, pour the custard through a fine-mesh sieve before continuing, but I never do.

Press a piece of plastic onto the surface of the custard and let it cool at room temperature or in the fridge until lukewarm. If you've got space outside on a cold day, this speeds the process up.

Take one wafer layer — I usually choose one with the sizing a little off — and chop it into breadcrumb-sized pieces. Transfer to a bowl and toss the crumbs with powdered sugar and set aside.

Place one of remaining wafer layers on your cake plate. Dollop 2/3 cup custard filling on it and use a spatula to spread it just a millimeter or so from the edges. It will seem very thick and wobbly — you're doing it right. Repeat with 6 more wafer layers and most of the remaining custard filling (I always have a small amount left; it never survives the afternoon), finishing with a final layer of custard. Let it hang out at room temperature for 10 minutes — just walk away — so it begins setting up.

When you come back, you'll see that some of the custard has spilled out the sides — it's totally fine, just scoop it up with your spatula and press it back over the sides, a bit like you're messily icing a cake. Sprinkle some of the powdered sugar wafer crumbs over the top, and then press small — you'll really have just enough — handfuls over the long sides. Transfer the napoleon to the fridge to rest overnight. The layers will absorb some custard and it will slice cleanly once they do. We find it takes 24 to 36 hours for the layers to soften to the ideal point.

The next day, dust with additional powdered sugar and cut into 1-inch slices. Leftovers keep for 4 to 5 days in the fridge.