Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper.
In a large bowl, beat the butter and sugar until light and creamy.
Add the egg and vanilla. Mix until smooth.
Add flour and salt. Mix just until a soft dough forms.
Fold in candied ginger or dried cranberries if using.
Roll dough into 1-inch balls and place on baking sheets.
Press a small thumbprint into the center of each cookie.
Fill each thumbprint with about ½ teaspoon hibiscus jam.
Bake for 12–14 minutes, until edges are lightly golden.
Cool on the tray for 5 minutes, then move to a rack.
