Combine chocolate and butter over a double boiler (low heat). Stir until all of the chocolate and butter have melted and are fully integrated. In a separate bowl, whisk together eggs and mustard, set aside. Place chocolate mixture in the bowl of a mixer with the paddle attachment. Add confectioners’ sugar and blend until fully incorporated. On low speed, add eggs slowly until a smooth batter has formed. Fold in flour with a spatula and be careful not to over mix. Place batter in greased 6 oz. ramekins and then place in the freezer for at least 4 hours. Bake at 350 until cakes are mostly cooked through (roughly 15 minutes). The center of the cakes should be liquid and you can tell they are ready when there is still about a quarter sized spot of liquid in the top center of the cakes. Allow to cool 5 minutes to unmold if desired.