Break the chocolate into individual squares and place in a bowl set over a pot of hot water (no hotter than 160°F.) on low heat. The water must not touch bottom of bowl. Add the oil.

Remove the pot from the heat and stir until the chocolate begins to melt. Return to the heat if the water cools, but be careful it does not get too hot. Stir until smooth. (The chocolate may be melted with the oil in a microwave oven if stirred every 15 seconds. Remove before fully melted and stir, using the residual heat to complete melting.)

Because of the milk solids in the white chocolate, the buttercream must be chilled and stirred to prevent seeding (the formation of tiny lumps). Fill a large bowl with ice cubes and water and sprinkle with 1 or 2 tablespoons salt. Fill a second bowl or the sink with very hot water. Set the bowl of buttercream in the ice water and stir constantly with whisk, until you just see whisk marks on the surface. Immediately place the bowl over the bowl of hot water to take off the chill. This will only take seconds. Feel the bottom of the bowl. It should feel cool not cold.

Allow the buttercream to sit for a few minutes, stirring occasionally with whisk. If it does not form peaks when the whisk is raised, chill again for a short time.