In a small saucepan, melt chocolate and butter over medium-low heat. Remove from heat; stir in corn syrup and 1 tablespoon (15 grams) water. Let stand until thickened, about 15 to 30 minutes. Use immediately, or cover and refrigerate for up to 4 days. Reheat over low heat before using.
Recipe by Bake from Scratch at https://bakefromscratch.com/amaretto-boston-cream-pie/