To make the sponge, in a small saucepan over low heat, heat the milk until it feels just warm, about 110 degrees. Pour the milk into the bowl of a stand mixer. Whisk in the yeast, then stir in the flour. Cover the bowl with plastic wrap and set aside at room temperature for about 30 minutes, until the sponge is well risen. Don’t allow more than 30 minutes for the sponge