1 tablespoon unsalted butter at room temperature and cut into
small pieces (plus extra butter to grease cans)
1/2 cup flour, sifted (plus extra to flour cans)
1 ounce white chocolate, finely chopped
1/4 cup boiling water
2 tablespoons sour cream, room temperature
The yolk of one large egg
1/2 teaspoon vanilla
1/3 cup sugar
1/8 teaspoon baking soda
1/8 teaspoon salt
Place rack in the center of the oven and preheat to 350 degrees.
Grease the insides of two 14-ounce cans and lightly dust them with flour, tapping out the excess. Place the cans on a baking sheet and set aside.
Place the white chocolate in a small microwave-safe bowl and heat until melted. Let the white chocolate cool to lukewarm.
Place the butter in a small microwave-safe bowl and heat until melted. Let the boiling water cool slightly. Add it to the bowl and whisk to blend. Add the sour cream, egg yolk and vanilla and whisk to combine.
Place the flour, sugar, baking soda and salt in a medium-size mixing bowl and whisk until blended. Add the sour cream mixture and the melted white chocolate and mix until blended and smooth.
Spoon the batter into the prepared cans, dividing it evenly between them. Bake the cakes until a toothpick in the center comes out clean, about 30 minutes.
Remove from the baking sheet and cool on a wire rack for 15 minutes. Gently run a thin, sharp knife around the edge of each can and invert them to release cakes. Place the cakes upright on the rack and cool.
To frost the cakes, cut each in half horizontally. Ice with flavor of choice.