Set the water, brown sugar and vanilla bean in a pot. Cook until it begins to boil and the sugar is dissolved, then lower to a simmer. Add the rhubarb and keep the heat on for 2 minutes. Turn off the heat and let cool to room temperature.
Make the muffin batter:
In a bowl, beat together the butter and brown sugar until light and fluffy, at least 3 minutes. Add the baking soda, powder, salt, vanilla and cinnamon and beat to combine. Add the eggs and beat until mixture is light and airy.
Add the flour and with the mixer on, slowly pour in the buttermilk and beat to combine.
Dice the rhubarb into small chunks and add it and the strawberries to the batter. Use a rubber spatula to gently fold them in.
Cover with plastic wrap and set in the fridge for 2 hours or overnight.
Bake the muffins:
Preheat the oven to 400 F. Line two muffin pans with muffin liners, alternating the liners placement - you’ll have six liners in each pan.
Scoop the muffin batter into the pan, each muffin cup will be nearly full with batter. Sprinkle ½ tablespoon of turbinado sugar on top.
Bake for 20-25 minutes until tops spring back showing a done middle.