Lemon Cupcake with Blackberry Buttercream

INGREDIENTS

• 1 cup white sugar
• 1/2 cup butter
• 2 eggs
• 1 1/2 teaspoons vanilla extract
• 1 1/2 cups all-purpose flour
• 1 3/4 teaspoons baking powder
• 1/2 cup low-fat milk
• 1 lemon, juice and zest
•  
• 1 cup butter, softened
• 1 teaspoon vanilla extract
• 1/4 teaspoon salt
• 4 cups confectioners' sugar
• 1/2 cup seedless blackberry jam