Coarsely chop the unsweetened chocolate.
Melt the butter and chocolate in a saucepan over low heat. Stir and melt until smooth.
Beat the eggs and sugar at medium speed with a stand mixer or a hand mixer. Beat until the mixture is well blended.
Once the chocolate is melted, turn the mixer down to low and slowly drizzle the chocolate into the mixture. Continue to blend until the chocolate is fully mixed in.
With the mixer still on low, gradually add the flour and vanilla until fully incorporated.