Let the temperature of the butter rise to room temperature. Heat it in the microwave at 500 W, (for 10 seconds if it's fall or spring, or 15 in winter) then blend together with flour (it's easy to do with the back of a spoon).
Add in the milk and microwave at 500 W for 1 minute and 40 seconds. Mix with a whisk. When it's thickened, it's done. Season with salt and pepper.
If you want it to be thin, heat the mixture for 30 seconds, mix vigorously, then repeat. Once it returns to room temperature, it will be thick and creamy but that's fine.
If you're making a large amount, blend the butter and flour well and heat up the milk before for 2 minutes in the microwave. Mix the milk in a little at a time and that should prevent lumps from forming. Then just keep heating for 30 seconds at a time.
You can make pasta sauce by mixing this, consommé, salt and pepper, nutmeg, curry powder, and herbs. Recipe ID: 2140040.
You can use it as pizza sauce Recipe ID: 2237103 and Recipe ID: 2177553 or gnocchi version Recipe ID: 2149921.
Story Behind this Recipe
I saw this in a book but I made it easily more delicious by adding some milk to it. Also, by making it in the microwave, it doesn't burn, it has a nice color, and it doesn't dirty many dishes making it quite handy! For the grams part of measurement, you can convert it as necessary. You can use 10 g flour, 10 g butter, and 150 ml milk (you can even adjust the amount of milk too). It'll go well in many different recipes.
Rather than make a regular basic sauce, I seasoned this with consommé and other flavours. If you don't cook it enough or cook it too much, you can put it right with more milk, water, and flour. If you half the amount of butter used, this becomes a low calorie recipe.