Pre-heat oven to 350 degrees. In a mixing bowl, add brownie mix, eggs, oil, and milk. Pour into a parchment or foil lined 9 inch baking pan. Bake for 30 minutes. Cool on a cooling rack. Refrigerate overnight.
Dough
Warm buttermilk in the microwave until slightly warm but not hot. Pour in a Kitchen aide bowl. Sprinkle yeast on top. Let it sit for 5 minutes. Add sugar, salt, vanilla, and egg. Attach dough hook and turn on the machine. Add the flour a cup at a time. After the flour is in, add the softened butter a tbsp at a time. Let the dough kneed for a few minutes. Place the dough in an oiled bowl, cover with plastic wrap and place in the refrigerator overnight. The next day, divide dough in half. Wrap one half in plastic wrap and place into a zip bag for another time. This would be enough dough for two babkas.Roll out the second half of dough into a rectangle.
Filling
Place dark chocolate in a microwave safe bowl with the butter. Microwave until melted. Add cocoa powder and powdered sugar and mix. Spread chocolate filling on top of dough until the entire area is covered. Take 1/3 of the brownies and chop into small pieces. Spread the brownie pieces all over the dough and filling. Roll dough up lengthwise. Cut down the middle so that you have 2 halves. Twist each half together, and place in a bread pan. Add some additional brownie pieces on top if desired. Let rise for an hour. Bake at 350 degrees for about 30 minutes or until browned. Cool on wire rack.
Ganache Glaze
Place chocolate and cream in a microwave safe bowl. Heat for 45 seconds-1 minute. Stir until smooth. Add additional milk if needed. Use a spoon to drizzle on babka.
