Are these really sugar-free? Nothing sweet goes in out of a bag: no caster sugar, no syrup, no sweetener. All the sweetness is the 480 g of ripe banana and 320 g of dates. That still leaves about 19 g of natural sugar in a square, so "no added sugar" is the more honest description than "sugar-free". What comes with it is 5 g of fibre and the potassium from the bananas.
Use bananas that look past it. Black-spotted and soft. This is the single biggest lever on how sweet the tray turns out.
Weigh the dates, do not count them. 320 g whatever variety you buy. Recipes that ask for a number of Medjools are written for American dates, and British shops rarely sell them that big. Medjool blend smoothest; Deglet Nour are drier and cheaper, so give them the full fifteen minutes in the hot water.
Blend the date paste completely smooth. Lumps do not disappear in the oven, they stay as chewy sweet pockets in the baked brownie. If the machine struggles, add a spoonful of the soaking water at a time until it moves.
Taste the paste before it goes in. If your batch of dates is on the mild side you will know at that point, and a few extra dates is the fix.
Do not overmix once the flour is in. Fold only until the dry patches disappear. Beating it develops the gluten and turns the crumb rubbery.
Trust the skewer, not the clock. A batter this wet can want longer than you expect, and an underbaked middle is the commonest complaint with brownies of this kind. If the skewer comes out with wet batter rather than moist crumbs, give it another five to ten minutes and test again.
Cool completely in the tin before cutting. Warm, they tear. Cold, they slice cleanly into sixteen squares.
Keeping them. Airtight for three days at room temperature, or up to a week in the fridge, where they firm up and slice beautifully. They freeze well for around three months: cut into squares first and layer them with baking parchment so they do not weld together.