Thai Massaman Curry with Beef (Kaeng Massaman)

0 Heat a Dutch oven, wok, or saute pan over medium heat. Add ¾ cup coconut milk (if using canned coconut milk that has separated, use the top, thicker portion of the milk) to the pan. Simmer coconut milk, stirring constantly, until it has reduced by half and you can see oil separating from the milk, 5 to 6 minutes. (Note: This step to “break” the coconut milk is part of the traditional process, but don’t worry if your coconut milk doesn’t show signs of separating after this initial cook time - just proceed with the recipe.)
1 Add curry paste and saute until fragrant, 2 to 3 minutes.
2 Add beef and stir to coat in the curry paste.
3 Pour water and 1 cup coconut milk over beef. 
4 Stir in potatoes, tamarind concentrate, palm sugar, and fish sauce.
5 Bring to a simmer and let curry simmer gently until beef is cooked through and potatoes are tender, 10 to 15 minutes. (Reduce heat as needed to prevent the curry from reaching a hard boil.)
6 Taste the curry and adjust the flavor, adding more sugar, tamarind, or fish sauce if needed. 
7 Serve curry over rice with peanuts and sliced jalapenos (if using) on top.