Prepare cake according to package instructions in a 9x13" pan. Remove cake from oven and cool for 15 to 20 minutes. When it has cooled, but is still warm, carefully poke holes evenly all over the cake with something round and about as thick as your pinky finger (like the handle of a wooden spoon).

In a medium bowl, whisk together the vanilla bean seeds if using and cold milk, then whisk in the instant vanilla pudding mix until just combined, but not thickened. Very quickly, before the pudding thickens, pour it over the cake and into all the holes, gently pressing it down into the holes if needed.

Refrigerate the cake for 1 hour to cool completely and thicken the pudding.

Place the chopped chocolate and butter into a heatproof bowl. In a medium saucepan warm the heavy cream over medium-low heat until it simmers. Gently simmer for 1 minute, stirring occasionally. Pour the hot cream over the chocolate and butter and whisk until the chocolate is melted and smooth. Let the mixture cool for 10 minutes, then spread it over the top of the cake.

Chill the cake for 2 to 4 hours until the glaze is set. You can also refrigerate overnight.