To prepare a the best butter mochi recipe, first you need to prepare the butter. First, you'll need the following ingredients: 1 cup (240 g) unsalted butter 2 tablespoons (60 ml) granulated sugar (optional) 12 teaspoon (0.5 tsp) vanilla extract Instructions: In the bowl of your electric mixer, beat the egg whites on medium speed until stiff peaks form. Gradually add the sugar mixture, beating well after each addition. Add the vanilla and mix until the mixture is light and fluffy. Fold in the whipped cream and sour cream. Pour the batter into the prepared pan and bake for 20 minutes. Remove the pan from the oven and let it cool for 5 minutes before serving.