Add cream and sugar to a heavy saucepan and let sit for a few minutes to help dissolve the sugar.

Set over medium to medium-high heat and bring to a gentle boil, stirring often. Add corn syrup and continue stirring.

While the mixture cooks, use a wet pastry brush to wash any sugar crystals from the sides of the pan.

Clip on a candy thermometer and cook until the mixture reaches 238°F, adjusting for your elevation as needed (see notes).

Carefully pour the hot syrup onto a damp marble slab, metal baking sheet, or chilled mixing bowl. Do not scrape the sides or bottom of the pan.

Place the butter on top of the hot candy and allow it to sit until the mixture cools to warm but not hot.

Beat the mixture (using a mixer with a dough hook or by hand) until it thickens, turns opaque, and becomes a creamy, dough-like fondant. Let it rest and cool completely.

Once the fondant is cool, knead in lemon oil or lemon extract. Start with a small amount and add more until the lemon flavor tastes slightly strong; the chocolate coating will soften the flavor.

For hand-dipped chocolates, roll portions of the lemon fondant into small balls or desired shapes. Chill on a parchment-lined tray until firm, then dip each piece into melted chocolate and place back on the tray to set.

For molded chocolates, coat the inside of each chocolate mold with melted chocolate, making sure the sides are fully covered. Chill until set. Pipe or spoon lemon cream into each cavity, leaving a little space at the top, then cover with more melted chocolate. Chill until fully set.

Remove chocolates from molds or parchment, trim edges if desired, and store in an airtight container in a cool, dry place.