Whisk the cornstarch with 1 cup of the cold water until smooth.
2
Bring the remaining 3 cups water to a simmer in a saucepan, then slowly whisk in the cornstarch mixture.
3
Cook, stirring, until thickened and translucent, about 5–10 minutes.
4
Remove from the heat and cool to 60–65°C (140–149°F), checking with a thermometer.
5
Stir in the diastatic malt powder.
6
Keep the mixture between 60–65°C for 90 minutes, stirring occasionally; do not let it boil.
7
Strain through a fine-mesh sieve lined with cheesecloth.
8
Return the liquid to a clean saucepan and simmer, uncovered, until it reaches about 107°C (225°F) or a pourable syrup consistency, usually 30–45 minutes.
9
Cool, transfer to a clean jar, and refrigerate.
10
This home method makes a corn-derived, maltose-rich syrup; its flavor and composition differ from commercial corn syrup.