Prepare the cake: Preheat the oven to 350℉. Butter and flour a 13x9 inch baking pan. In a large bowl, use an electric mixer to combine the butter and sugar until light and fluffy, 1 to 2 minutes. Add the eggs one at a time, beating after each addition. Blend in the melted chocolate and vanilla. Mix in half the flour and pinch of salt, then the buttermilk, then the rest of the flour, and then the soda/vinegar. Scrape the batter into the prepared pan. Bake 25 to 30 minutes.
Prepare the glaze: Melt the butter in a heavy saucepan over medium-low heat. Add the remaining ingredients, powdered sugar ½ cup at a time. Pour the warm glaze over the cake as soon as you remove it from the heat. Add sprinkles, if you want them!
Let the cake set for a little while before cutting into it.