Combine the eggs, sugar, and salt in a 20-qt mixer bowl. Place over a hot water bath and bring the contents of the bowl up to 48°C / 120°F while whisking slowly (don’t whisk too vigorously or else too much air will be incorporated and it will take too long for the ingredients to warm up). The point of this step is to dissolve the sugar and salt and to warm up the eggs so they will whip quickly.
Transfer the bowl to a mixer and whip on high speed for 5 minutes, then turn the speed down to medium and whip for 15 more minutes.
Meanwhile, preheat a convection oven to 176°C / 350°F, open the vents, and turn the fan speed down to low.
Grease a full-size sheet pan and line it with parchment paper or a nonstick rubber mat. If using the mat, grease only the border of the pan.
Sift the bread flour, pastry flour, and coffee powder together twice.
After the eggs have whipped to full volume, take the bowl off the mixer and gently fold in the dry ingredients.
Fold in the butter carefully so as not to deflate the génoise.
Immediately after, pour the batter into the prepared sheet pan and spread out evenly with an offset spatula. Bake until golden brown and the sponge springs back when gentle pressure is applied, about 12 minutes.
Cool to room temperature.
Freeze the génoise for 2 hours and then cut into rectangles 2.5cm / 1in by 7.5cm / 3 in.
Reserve in an airtight container at room temperature. Discard any leftover génoise after service.