If using canned dulce de leche, skip to step 3. To make the homemade dulce de leche: in a heavy 3- to 4-quart saucepan combine 4 cups whole milk, 1 1/4 cups sugar, and 1/4 teaspoon baking soda. Bring to a boil over medium-high heat, then reduce to a gentle simmer and cook uncovered, stirring occasionally. After about 1 hour stir more often to prevent scorching; continue simmering until the mixture is caramelized, thickened, and a deep golden color, about 1 1/2 to 2 hours. Remove from heat and stir in 1 teaspoon vanilla. Transfer to a bowl and let cool to room temperature.

Measure 1 1/2 cups of the cooled homemade dulce de leche for the mousse. (If using canned dulce de leche, measure 1 1/2 cups from the can.)

In a heavy medium saucepan, combine 1 1/2 cups whole milk and the 1 1/2 cups dulce de leche. Heat over medium, stirring constantly, until the mixture is smooth and hot but not boiling.

Remove the pan from the heat, add 6 ounces finely chopped bittersweet chocolate, and whisk until the chocolate is completely melted and the mixture is smooth. Set aside to cool until it is no longer hot to the touch (cool to lukewarm or room temperature).

In a medium bowl, beat 1 cup heavy cream with an electric mixer until soft peaks form. Gently fold the whipped cream into the cooled chocolate–dulce mixture in two additions, folding just until combined and no streaks remain.

Divide the mousse among six serving glasses, cover, and refrigerate until firm, about 4 hours. Top with whipped cream if desired before serving.