Everyone makes this once, it is wonderful for a week, and by the third week it tastes faintly bitter and gets quietly thrown away. The recipe is not the problem.
Short answer: Grind the nuts cold and in short pulses, and keep the powder in the fridge. Ground almond is exposed oil, and a jar on a warm shelf is bitter within three weeks - which is why most homemade milk masala disappoints by the second week.

Why katran instead of whole almonds
Badam katran is almond slices - the offcuts from slicing almonds for mithai. They are the same almond, already cut, and cost less per kilo than whole grades.
Since you are about to grind them to a powder, there is no argument for buying whole. The only thing a whole almond gives you here is the chance to peel it, and for milk masala the skins are usually left on anyway - they darken the powder slightly and add almost nothing bitter in this quantity.
The same logic gives you kaju tukda rather than whole cashew. The only item worth buying at grade is the pista, and that is for colour: Pishori kernels are greener, and a milk masala that looks pale grey is harder to enjoy than one flecked with green.

The grinding, which is where it goes wrong
Nuts are about half oil. Grinding them generates heat, heat releases that oil, and released oil does two things: it turns the powder into a paste, and it starts oxidising immediately.
So: chill the nuts for twenty minutes first. Grind in short pulses with rests between. Stop the moment it is a powder - the instinct to run it another ten seconds for a finer texture is what turns it to paste.
Sieve, and send anything coarse back for one more pulse rather than grinding the whole lot longer.
The cardamom goes in at the very end, with two or three pulses only. Grinding elaichi with the nuts from the start means grinding it for far longer than it needs and losing most of its aroma to the heat.

Keeping it
Airtight jar, in the fridge. Two months at full quality. On a kitchen shelf in a Salumbar summer, three weeks before the bitterness starts.
If you add sugar to the mix, it keeps less well - sugar draws moisture and the powder cakes. Better to keep the masala unsweetened and sweeten each glass, which also lets different people in the house have it their own way.
Make 200 grams at a time rather than a kilo. It is fifteen minutes of work and the difference between the first glass and the last is the whole point of making it yourself.
Making the glass
Two to three teaspoons per glass of milk. Add the powder to cold milk and whisk, then heat - adding powder to hot milk gives lumps that never quite disperse.
Simmer gently for two or three minutes rather than boiling hard. The nuts need a little time to release into the milk, and a hard boil drives off the cardamom.
Kesar, if you are adding extra, goes in soaked in a spoon of warm milk at the end.
Cold badam milk is also excellent and is the better version through summer - make it up, chill it for a few hours, and the texture thickens as it sits.

Buying it in Salumbar
Our almonds (badam) is on this site with the weight and the price on every pack. Order before 5 PM and it reaches you the same day inside Salumbar, free over ₹200. We have been on Sadar Bazar since 1961. If you would rather just say what you want, send it on WhatsApp and we will confirm the price before we pack anything.
Questions people ask
Why did my milk masala turn bitter?
The ground nuts went rancid. Ground nut is exposed oil and oxidises fast - three weeks on a warm shelf is typical. Keep it in the fridge and make small batches.
Which almonds for badam milk?
Katran - almond slices - since it is being ground anyway. Same almond, lower price than whole grades.
Why does my powder turn into a paste?
Over-grinding. The heat releases the oil. Chill the nuts first and grind in short pulses, stopping as soon as it is a powder.
Should I add sugar to the mix?
Better not to. Sugar draws moisture and the powder cakes, and it keeps less well. Sweeten each glass instead.
How much powder per glass?
Two to three teaspoons. Whisk into cold milk before heating, or it lumps.
How long does homemade milk masala keep?
Two months in an airtight jar in the fridge. About three weeks at room temperature in summer.