Prepare the Butterscotch:

1. Add butter to a medium pot and turn on the heat to medium. Once butter starts to melt, add in brown sugar. Using a spatula, carefully stir the sugar mixture around to evenly distribute the heat and ensure even melting of the sugar. Cook for about 3-4 minutes, or until the edges of the sugar start to bubble up.
2. Carefully and slowly, whisk in the heavy cream, about ¼ cup at a time. Continue until all of the cream is mixed in.
3. Continue to cook the butterscotch over medium heat, watching carefully to ensure it doesn’t boil over, for 5 minutes, or until slightly thickened.
4. Remove the butterscotch from the heat, and let it sit for 5 minutes to cool. Set aside 2 tablespoons of the butterscotch to drizzle on top of the pudding for serving.

Make the pudding:

1. Add cornstarch, salt and egg yolks to a medium bowl and whisk together until thoroughly combined. Then whisk in the milk.
2. Next, temper the egg mixture: Slowly pour 1 cup of the hot butterscotch into the milk/egg mixture, while gently whisking. Then, slowly pour the whole egg mixture into the pot of butterscotch while gently whisking.
3. Turn on the heat to medium and cook, whisking constantly, and occasionally taking a spatula and scraping up any thickened pudding in the edges of the pan. Continue to cook until the pudding has completely thickened and large bubbles are forming and popping, about 8 minutes. 
4. Remove from heat; immediately pour through a fine mesh strainer into a bowl, using a spatula to push all the pudding out while leaving behind any bits of cooked egg. Whisk the extract and butter into the pudding. 
5. Pour the pudding into serving containers or a bowl. 
6. Serve slightly warm, or cold with whipped cream and desired toppings.