Cream butter with egg. Add sugar and mix well. Sieve dry ingredients and add to butter mixture while also pouring milk. Add vanilla. Carefully add blueberries mixture (fold with the dough).
Set into a buttered springform mold and oven bake at 350 °F (180 °C) for about 40 minutes.
Meanwhile, melt butter in a small cauldron. Add brown sugar and cream and simmer for a few minutes. Cool down. When out of the oven, verify with a skewer its inner temperature and pour sugar cream fudge all over.