To prepare the truffles, heat the cream to a simmer and pour it over the chopped chocolate, Let it sit for a minute then gently whisk to blend. Chill this until set, about 2 hours. Spoon this ganache into 20 truffles, shaping then into ¾-inch balls between your hands. Chill the truffles until ready to use.
Preheat the oven to 180°C and line 20 muffin cups with paper liners.
Melt the chocolate and butter together in a saucepot, stirring over low heat, then stir in 2/3 cup of the brown sugar and 1/3 cup of the milk, stir over medium heat until the sugar has dissolves (don’t worry if the mixture does not look smooth – it will even out later). Pour this mixer into a bowl and whisk in the remaining 2/3 cup of sugar and cool to room temperature. Whisk in the eggs one at a time, blending well after each addition. Whisk in the vanilla.
In a separate bowl, sift the flour, baking soda and salt. Add this to the chocolate mixture in 3 additions alternately with the milk, starting and ending with the flour and whisking well after each addition (the batter will be quite fluid). Pour the batter into the prepared muffin cups, filling to halfway.
Gently place a truffle in the centre of each cupcake – do not press it in as it will settled and become completely enveloped by the batter as it bakes. Bake the cupcakes for about 20 minutes, until the cake springs back when gently pressed. Cool the cupcakes completely before frosting.
For the frosting, melt the chocolate and butter in a saucepot over low heat, stirring until melted. Transfer this to a bowl and whisk in the sour cream and vanilla. Beat in the icing sugar until the mixture is smooth and spreadable, adding a touch more icing sugar if needed. Spread or pipe the frosting on each cupcake.
The cupcakes should be stored at room temperature in an airtight container and will keep for up to 2 days. Do not refrigerate.
NOTE: If consumed within 24 hours of baking, the truffle filling actually stays soft and almost fluid. After a day, the truffle sets up further, but is still satisfyingly delicate.