Chocolate Toffee Trifle

1 1/2 ounces unsweetened chocolate, finely chopped
1 cup sifted cake flour
1 teaspoon baking soda
1/4 teaspoon salt
1 3/4 cups (packed) golden brown sugar
1/2 cup (1 stick) unsalted butter, room temperature
1 large egg
1 large egg yolk
1/4 cup buttermilk
1/2 cup boiling water
1 teaspoon vanilla extract
3 cups chilled whipping cream
1 1/2 cups chocolate-covered English toffee bits (two 6 ounce bags)
Semisweet chocolate shavings

Heat oven to 375 degrees F. Butter and flour 9-inch square baking pan with 2-inch-high sides.

Melt 1 1/2 ounces chocolate in top of double boiler set over simmering water, stirring until smooth. Remove from over water. Let cool 10 minutes.

Sift flour, baking soda and salt into medium bowl. Beat brown sugar and butter in large bowl until blended. Add egg; beat well. Beat in yolk. Add melted chocolate; beat until blended. Mix in dry ingredients alternately with buttermilk, beginning and ending with dry ingredients. Add 1/2 cup boiling water and vanilla extract; stir until blended (batter will be thin.)

Pour batter into prepared pan. Bake until tester inserted into center comes out clean, about 35 minutes. Transfer pan to rack and cool completely. (Can be prepared 1 day ahead. Cover with foil and store at room temperature.)

Beat cream in large bowl until stiff peaks form. Spoon 1 1/2 cups whipped cream into bottom of large glass trifle bowl. Sprinkle with 1/2 cup toffee bits. Crumble 1/3 of cake over toffee. Repeat layering of whipped cream, toffee bits and cake, creating 3 layers of each. Spoon remaining whipped cream over top, spreading to cover. Cover trifle; refrigerate at least 4 hours. (Trifle can be prepared 1 day ahead. Keep refrigerated.)

Sprinkle trifle with chocolate shavings and serve.

Serves 10 to 12.