Gather all the ingredients. Use a kitchen scale to weighthe butter and flour. Depending on how you scoop the flour, the weight can vary. If you don't have a kitchen scale, see this post for how to measure flour.
To Make a Brown Roux
In a small saucepan, melt the butter over low to medium-low heat (you can also cut the butter into small pieces first).
When the butter is completely melted, add the flour. With a blunt-end wooden spatula, stir to combine the butter and flour.
Soon, the butter and flour will fuse and swell. Cook for 20-25 minutes over low heat. Stir constantly because the roux burns easily. If the roux starts separating, switch to a whisk and mix vigorously.
The roux will turn golden brown. You can even go for a bit darker color (watch my video).
To Make the Curry Roux
Add the curry powder, garam masala, and cayenne pepper (optional) to the roux.
Cook and stir for 30 seconds and remove from the heat. You can use the roux immediately if you'd like (see the instructions below).
To Shape the Roux into a Block (This is an overnight process)
Transfer the roux to a glass or metal container lined with parchment paper. Let it cool completely on the kitchen counter, then refrigerate to solidify overnight.
On the following day, take out the curry block from the container. If the curry block gets stuck to the container, release it with a butter knife or an offset spatula.
With a sharp knife, cut the block into eight 1-inch (2.5 cm) cubes. Store the cubes in a glass container.
Store the container with curry cubes in the refrigerator for 1 month and in the freezer for 3-4 months. Use them soon before they lose their flavor and aroma.
To Use in a Curry Recipe
Very important! Please do not forget to season your final dish with salt, as the homemade roux is not salted. I usually add about 1 Tbsp salt to the broth (ranges from 2 tsp to 4 tsp depending on the recipe). Store-bought roux is quite salty, so you may want to add more salt to achieve a similar taste.
Add the curry roux mixture (before solidifying) or cubes to the broth of your curry recipe (follow the recipe instructions on when to add the roux).
How much mixture or how many cubes should we use? I think roughly 6-7 cubes are equivalent to one box of store-bought curry roux (that requires 4 cups or 1L of broth/water). Simmer over low heat for 5-10 minutes. Heat will thicken the roux.
If the curry flavor is lacking or the curry does not thicken, add more roux. When you add meat and/or veggies, they release more moisture to the broth; therefore, you will need to adjust the amount of roux you use according to what you’re cooking.