Steps:

• Preheat oven to 350°. Line bottoms of three 6-in. round baking pans with parchment; grease and flour pans., In a large bowl, whisk flour, sugar, baking powder and salt. Beat in butter until crumbly. Add egg whites, 1 at a time, beating well after each addition. Gradually beat in 1/4 cup milk and extracts; beat on medium until light and fluffy, about 2 minutes. Gradually beat in remaining milk. Gently fold in jimmies., Transfer batter to prepared pans. Bake until a toothpick inserted in each center comes out clean, 25-30 minutes. Cool in pans 10 minutes before removing to wire racks; remove parchment. Cool completely., For buttercream, in a microwave, melt chocolate with cream until smooth, stirring every 30 seconds. Set aside to cool slightly. In heatproof bowl of stand mixer, whisk egg whites, sugar, cream of tartar and salt until blended. Place over simmering water in a large saucepan over medium heat. Whisking constantly, heat mixture until a thermometer reads 160°, 8-10 minutes., Remove from heat. With whisk attachment of stand mixer, beat on high speed until cooled to 90°, about 7 minutes. Gradually beat in butter, a few tablespoons at a time, on medium speed until smooth; beat in vanilla and white chocolate mixture., Spread frosting between layers and over top and sides of cake. Divide remaining buttercream into smaller portions; stir in food coloring to achieve desired colors. Decorate as desired. Store in refrigerator.