In a small saucepan, combine the water and sugar. Bring to a gentle simmer until the sugar has dissolved.
Remove from the heat and add the basil leaves. Cover and steep for 20 to 30 minutes. Strain out the leaves and transfer the syrup to a jar.
Stir the basil simple syrup into the milk to make the basil milk.
Add the strawberries, sugar, and water to a small saucepan. Cook over medium-low heat for 8 to 10 minutes, stirring occasionally, until thick and jammy.
Allow the mixture to cool completely, then blend or mash until mostly smooth.
Combine 2 tablespoons of the strawberry puree with the heavy cream and froth until thick and airy.
Fill a tall glass with ice. Pour in the basil milk, then add the espresso and water.
Top generously with the strawberry foam. Garnish with a fresh basil leaf and a sliced strawberry if desired. Serve immediately.
Notes
Basil syrup keeps up to two weeks in the fridge. Oat or almond milk work; strong brewed coffee or cold brew concentrate can replace espresso.