"Top Tier" Devil's Food Cake with Sour Cream-Fudge Frosting

INGREDIENTS

• 1/2 teaspoon vanilla extract
• 1/4 cup natural unsweetened cocoa powder
• 1/4 cup (1/2 stick) unsalted butter, room temperature
• 2/3 cup cake flour
• 1 large egg
• 1/4 teaspoon salt
• 2/3 cup sour cream
• 1/4 cup boiling water
• 1/4 cup sugar
• 1/4 cup buttermilk
• 1/2 teaspoon baking soda
• 8 ounces high-quality milk chocolate (such as Lindt, Perugina, or Valrhona), chopped
• 1 ounce high-quality milk chocolate (such as Lindt, Perugina, or Valrhona), chopped
• 4 teaspoons light corn syrup
• 1/4 cup canola oil
• Organic roses (for garnish)
• 1/3 cup (packed) dark brown sugar