Liberally butter two 6-by-3-inch cake pans and dust with flour. Tap out the excess flour. Preheat the oven to 350 degrees F. Sift together the flour, baking powder, and salt into a bowl and set aside. In a saucepan over medium-low heat, combine the butter and milk and heat, stirring occasionally, until the butter is melted, 3 to 4 minutes. Alternatively, in a microwave-safe bowl, combine the butter and milk and microwave on high at 10-second intervals, stirring between each, until the butter is melted. Pour the milk mixture into a large bowl. Set aside and let cool to slightly above room temperature; an instant-read thermometer should register between 80 and 85 degrees F. You want the milk and butter to remain incorporated, so whisk the mixture occasionally and vigorously while it is cooling and up until you add it to the batter. In the top pan of a double boiler or in a bowl for a bain-marie, whisk together the sugar, eggs, and vanilla. Fit the top pan or bowl over (but not touching) simmering water in the bottom pan or a saucepan and gently warm the mixture, stirring, just until the sugar is dissolved and the thermometer registers 110 degrees F, about 10 minutes. Pour the egg mixture through a medium-mesh sieve into the bowl of a stand mixer. Fit the mixer with the whisk attachment and whisk on high speed until the mixture is light and fluffy and has cooled to slightly above room temperature (between 80 and 85 degrees F), about 10 minutes. (Note: The batter becomes more stable as the mixture cools; this is an important factor in creating a cake with good structure.) Stop the mixer. Add the sifted dry ingredients and mix until just incorporated, preferably by hand, lifting and folding in from the bottom center. Or, use the mixer set on low and whisk just until incorporated, about 5 seconds. Using a rubber spatula, scrape down the sides of the bowl and mix again just briefly by hand or machine. The batter should be smooth. Check the milk mixture to make sure it is fully cooled, then slowly pour it into the batter, whisking on low speed. Stop mixing as soon as the batter is well blended and smooth. Remove the bowl from the mixer, scrape down the sides of the bowl again, and fold by hand a few times with the spatula. Divide the batter between the prepared pans. Bake until the cakes are golden brown and a tester inserted in the center comes out clean, 30 to 35 minutes. Transfer to wire racks and let cool in the pans for about 20 minutes. When the cakes are cooled enough to handle the pans but still a tad warm to the touch, carefully run an offset spatula around the edges of the pans to loosen them, then invert the cakes onto the racks and remove the pans. Let cool for about 30 minutes longer. Wrap the cakes tightly in plastic wrap and refrigerate to ensure that the interiors are completely cooled before decorating, at least 1 hour or for up to 3 days. To freeze, wrap tightly in a second layer of plastic wrap and store in the freezer for up to 2 months.
Reprinted with permission from Miette by Meg Ray with Leslie Jonath, Chronicle Books copyright (c) 2011