Combine egg yolks, sugar, kosher salt, and corn starch in a heat-proof bowl. Whisk thoroughly until pale and fluffy.
In a medium stock pot, heat lemongrass in 2 tbsp butter over medium heat until softened. Turn heat up to medium-high and add heavy cream and coconut milk.
Blend lemongrass and creams in a high speed, heat-proof blender (or with an immersion blender) for 1-2 minutes.
Strain cream into a clean sauce pot through a fine mesh sieve or chinois. Heat until bubbling over medium heat
Remove cream from heat and slowly incorporate into the egg yolk mixture, whisking constantly. Once all the liquid is added, transfer it all back to the sauce pot.
Heat cream mixture over medium-high heat and stir constantly for 5-10 minutes until thickened. Remove from hat and add 6 tbsp cold butter.
Let pastry cream cool in an air-tight container for at least 30-60 minutes to firm up.
Preheat oven to 425°F. Roll puff pastry into 2 large sheets, about 15" x 10". Cut into 3 large rectangles each.
Spoon pastry cream in the middle of each rectangle, leaving a 1/2" border on each. Then, cut rhubarb down to the size of the inside rectangle and place three stalks on each pastry.
Brush each trio of rhubarb with maple syrup. Bake for 20-25 minutes until golden brown.