Whisk together the larger amount of bread flour, yeast, malt syrup, sugar, and salt in a medium bowl.
Sprinkle the mixture over the sponge. Do not stir. The layer of dry ingredients will protect the sponge from drying out while it goes through fermentation. Cover the bowl with plastic wrap and let sit at room temperature for 1 to 4 hours. Again, the longer the dough ferments, the more flavor the dough will have.
It is likely the sponge will bubble through the flour layer as it ferments. This is expected.
Next, add the butter to the stand mixer bowl and 1 Tbsp (9g) of the remaining flour (i.e., part of the “smaller amount”).
Mix these ingredients on low speed using the dough hook attachment until the flour is moistened (approximately 1 minute).
Increase the speed to medium and knead for 5-7 minutes. If the dough does not pull away by the end of the kneading time or if the dough still feels a little tacky/sticky, add the remaining flour.
The dough should be smooth and elastic. It should spring back immediately when gently pressed with your finger.