Butterscotch Cupcakes - Cupcake Fanatic

Ingredients

• 1/4 cup (57 g) dark brown sugar
• 1/4 cup (59 ml) water
• 2 teaspoons (10 ml) white or dark rum
• 1/4 cup (57 g) unsalted butter
• 1 cup (228 g) packed light or dark brown sugar
• 3/4 cup (180 ml) whipping cream (35%)
• 2 teaspoons (10 ml) pure vanilla extract
• 1/2 teaspoon (4 g) sea salt or fleur de sel
• 1-3/4 cups (236 g) all-purpose flour, sifted
• 1/2 cup (57 g) cake flour, sifted
• 1 cup (230 g) packed dark brown sugar
• 2-1/2 teaspoons (13 g) baking powder
• 1/2 teaspoon (4 g) salt
• 1 cup (227 g) unsalted butter, cold and cut into small even pieces
• 2/3 cup (160 ml) milk, at room temperature
• 1 teaspoon (5 ml) lemon juice
• 3 eggs, at room temperature
• 1 cup (240 ml) whipping cream (35%), cold
• 3/4 cup (94 g) confectioners’ sugar
• 1 cup (240 ml) mascarpone cheese, softened
• Toffee bits for sprinkling