Melt butter in a pan over medium-high heat (8/9 out of 14 on my stove).

Add curry to the butter, so it gets toasted. This enhances the curry flavor significantly. It should only be toasted for about 20-30 seconds and definitely should not burn.

Add the flour to the pan and whisk well, so you don't get lumps and it's well incorporated. This prevents the sauce from tasting like flour.

Now, gradually add the liquid a little at a time. Essentially, you could only use milk, but if you're, for example, boiling potatoes at the same time, I typically use 1-2 dl of the cooking water from that. However, I never use just cooking water exclusively. If you have stock or bouillon, I think half of the liquid should come from that. If you only use milk, you don't need to use cream.

Continue to add liquid (possibly including the cream) until the sauce reaches the desired consistency.

Remember that the sauce thickens as it cools, so it doesn't need to be too thick.

Season the sauce with salt, pepper, a little vinegar, and sugar (possibly gastrique), and it's ready to serve.

Bon appétit.