Preheat your oven to 350°F (175°C). Line a cupcake pan with cupcake liners.
In a large mixing bowl, combine flour, cocoa powder, sugar, baking powder, and salt. In another bowl, mix eggs, milk, melted butter, and vanilla extract. Gradually mix wet ingredients into dry until just combined.
Gently fold in the chopped dark chocolate.
Spoon the batter into the cupcake liners, filling them about 2/3 full. Bake for 18-20 minutes.
Let the cupcakes cool in the pan for about 5 minutes, then transfer to a wire rack to cool completely.
Beat softened butter until creamy. Gradually add powdered sugar, then mix in melted white chocolate and milk until smooth.
Frost each cupcake with the white chocolate buttercream.
Drizzle with salted caramel sauce and sprinkle with toffee bits.
