Instead of gelatin sheets, use powdered gelatin which blooms and dissolves quickly.
Measure gelatin according to recipe and bloom it in 3 tablespoons of cold water for 5-10 minutes.
Once you’ve made the panna cotta cream mixture, dissolve the bloomed gelatin while the mixture is hot.
After dissolving gelatin, strain the mixture to avoid lumps.
To have the perfect texture, chill panna cotta for 4-6 hours before serving.