Preheat oven to 350°. Line bottoms of 3 greased 8-in. round baking pans with parchment; grease paper. In a large bowl, cream butter, oil and sugar until light and fluffy, 5-7 minutes. Add eggs, then egg whites, 1 at a time, beating well after each addition. Beat in vanilla. In another bowl, whisk flour, baking powder and salt; add to creamed mixture alternately with buttermilk, beating well after each addition. Fold in sprinkles.
Transfer to prepared pans. Bake until a toothpick inserted in center comes out clean, 20-25 minutes. Cool in pans 10 minutes before removing to wire racks to cool completely.
For buttercream, in a large microwave-safe bowl, combine chocolate chips and cream. Microwave on high 30 seconds. Stir gently. Continue microwaving on high, stirring every 30 seconds, until melted and blended; cool to room temperature. In another large bowl, beat butter until creamy. Gradually beat in confectioners’ sugar until smooth. Add vanilla and salt. Beat in cooled chocolate. Beat until light and fluffy, about 5 minutes.
To assemble cake, place 1 cake layer on a serving plate; spread with 3/4 cup frosting. Repeat layers. Top with remaining cake layer. Frost top and sides of cake with 1/2 cup frosting. Freeze until set, about 10 minutes. Frost top and sides with remaining frosting.