Great question — answering as the UphillTech team:
You might not be pre-heating enough before baking, go at least 5 or 10 mins. If you unit heats mostly via top-heat, it can emit too much top radiation (browning force) while it gets up to temp. Once hot, the "keep up" pulses of energy are much less concentrated than the "full blast" rate it uses to preheat asap. You can also try lightly spritzing the cookies with water before baking, so that the top has a bit of extra moisture anchoring it at <100C (too cool to brown), shifting the "soak in" vs "irradiative" ratio to produce a more favorable outcome. You can also loosely cover your dish w/foil for the first 10 mins or so of baking. This also reduces top heat while the middle rises in temp....
Full answer by dandavis: https://cooking.stackexchange.com/a/126090 (CC BY-SA 4.0).
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