for the cupcakes
4 1/2 ounces high-quality bittersweet chocolate, finely chopped
9 tablespoons (1 stick plus 1 tablespoon) unsalted butter, room temperature
1 cup confectioners’ sugar
6 large eggs, separated, at room temperature
1 teaspoon vanilla extract
1/2 cup granulated sugar
1 cup all-purpose flour
For the filling
3/4 cup toffee, smashed into small pieces
3/4 cup caramel sauce
For the frosting
1 cup (2 sticks) unsalted butter, room temperature
2 1/2 cups powdered sugar
1/3 cup carmel sauce
Seeds scraped from 1 split vanilla bean or 1 1/2 teaspoons vanilla extract
1/8 teaspoon salt
Pieces of toffee (Heath Bar, Skor Bars, etc) for decorating
Make your cupcakes (can be made up to 2 days in advance): Preheat oven to 400F. Prepare a muffin pan with lines.
In the top part of a double boiler over very hot, but not simmering, water, or in a microwave at medium power, melt the chocolate. Remove from the heat or the oven, and let stand, stirring often, until cool.
Beat the butter in the bowl of a stand mixer fitted with the paddle blade on medium-high speed until smooth, about 1 minute. On low speed, beat in the confectioners’ sugar. Return the speed to medium-high and beat until light in color and texture, about 2 minutes. Beat in the egg yolks, one at a time, scraping down the sides of the bowl. Beat in the chocolate and vanilla.
Beat the egg whites and granulated sugar in a large bowl with an electric mixer on high speed just until they form soft, shiny peaks. Do not overbeat. Stir about one fourth of the beaten whites into the chocolate mixture to lighten it, then fold in the remaining whites, leaving a few visible wisps of whites. Sift half of the flour over the chocolate mixture, and fold in with a rubber spatula. Repeat with the remaining flour.
Fill the lined muffin cups three-quarters full, being careful not to overfill. Bake until a toothpick inserted in the center of the cupcake comes out clean, about 20-22 minutes. Cool the cupcakes in the pan for 5 minutes, then transfer to a wire rack to cool completely.
Once cool, prepare your filling, by mixing together toffee pieces and caramel sauce.
To make the frosting: In the bowl of a stand mixer, beat butter and caramel sauce until smooth. Add sugar, seeds from vanilla bean, and salt. Beat on medium-low speed until blended, scraping down sides of bowl. Increase to medium-high and beat until light and fluffy.
To assemble: Insert a small knife at a 45 degree angle about 1/8 inch from the edge of each cupcake and cut all the way around, remove a cone of cake. Cut away all but the top 1/4 inch of the cone; leaving only a small disk of cake which will be used to top the cupcake. Fill each cupcake with 2 tablespoons of the filling and top with the disk of cake. Pipe frosting onto cooled cupcakes. Drizzle with caramel sauce and decorate with additional toffee pieces.