• Gather all the ingredients. Use a kitchen scale to weigh the butter and flour.
  

Make the roux

• In a small saucepan, melt the butter over low to medium-low heat (you can cut the butter into small pieces first).
  
• Once the butter is fully melted, add the flour. Stir with a silicone-tipped wooden spatula to combine.
  
• The mixture will come together and puff slightly. Cook over low heat for 20-25 minutes, stirring constantly, as roux burns easily. If it starts to separate, switch to a whisk and beat vigorously.
• The roux will turn golden. You can even go for a slightly darker color if you like.
  

Make the Japanese curry roux

• Add the curry powder, garam masala, and cayenne pepper to the roux.
  
• Cook and stir for 30 seconds, then remove from the heat. You can use the roux immediately if you like (see the instructions below).
  

Make roux blocks

• Transfer the roux to a glass or metal container lined with parchment paper. I like to use handy aluminum tins.
  
• Let it cool completely at room temperature, then refrigerate overnight to set.
• The next day, remove the curry block from the container. If it sticks to the container, loosen it with a butter knife or spatula.
  
• Using a sharp knife, cut the block into eight 2.5-cm cubes. Store the cubes in a glass container.

Use in a Japanese curry recipe

• Add the roux mixture (before it sets) or the cubes to the broth for your curry (follow the recipe for when to add the roux).
• How much roux or how many cubes should you use? About 6 to 7 cubes are equivalent to one box of store-bought curry roux (enough for 1 liter of broth or water).
• Simmer over low heat for 5 to 10 minutes, until the roux thickens.