Luca Montersino's basic choux buns

I put water, milk, small butter pieces and salt in a heavy-bottomed pan.
I boil and add all flour at once to avoid lumps.
I stir and dry over heat a few minutes until releasing from sides.
I transfer to a bowl for electric beaters or mixer with whisk, spread to cool slightly, mix a minute and add eggs 1 at a time, absorbing each before the next.
I continue until all eggs are added.
Once creamy but not too smooth, it is ready. If still stiff, beat another whole egg and add gradually to the right consistency.
I put in a piping bag with plain nozzle and heat conventional oven to 220°C.
I lightly butter a tray and wipe excess. Montersino advises no parchment, so buns grip the tray and rise upwards rather than bulge below. I pipe walnut-sized buns.
I bake at 220°C for 15 minutes, then 190 for another 15 minutes, watching browning. I switch off, prop door slightly open with a wooden spoon to release steam and cool.
Store cold unfilled buns in tins.