Use sweet bread (bread with a high sugar content) for this recipe. In Thailand it's very difficult to find bread that isn't sweet, even a cheese sandwich in Thailand is typically made from sweetened bread and sweet cheese with an added layer of sweet mayonnaise. I'm afraid that is how Thailand is heading these days, copying western food, but adding more sugar to it.
Pandan (aka Bituoy) is a green leaf with a candy/bubblegum flavour used to make flavoured water with is then used for recipes like this one. You can also buy pandan flavouring in Asian shops if you can't find the leaf. Alternatively as a fall back, use vanilla essence and make vanilla toast.
I've recorded a video of this toast being sold in Bangnarmphueng Floating Market Bangkok that you can see below:
Ingredients for Topping
200 ml Evaporated Milk
100 ml Coconut Milk
1 Tablespoon Wheat Flour
1/2 Teaspoon Salt
5 Tablespoons Sugar
2-3 Tablespoons Pandan/Bitouy Water
100 gms Butter
4-5 Thich Slices of Bread
1. Mix all the ingredients for the topping together in a saucepan and cook through very slowly until it thickens, stirring all the time. You want to ensure the flour is cooked through.
2. Take 4-5 thick slabs of bread, toast them lightly on each side.
3. Spread melted butter all over one side of the toast, make sure it is completely covered.
4. Grill the bread a little more butter side up, to make sure the butter is melted and soaked into the bread, then flip them over and grill butter side down to slightly cook the butter.
5. Pipe the warm pandan sauce over the bread. You can also spread it, or spoon it over.
6. Serve immediately.