Line the bottom and sides of a 9-inch square pan with parchment. Set a whisk, heatproof spatula, and candy thermometer beside the stove. Keep all equipment dry.

Into a large, deep saucepan add 350 g sugar, 160-170 g golden syrup, 120 mL water, and 1/2 tsp salt (or 1/4 tsp). Stir gently only to wet the sugar.

Heat over medium-high, swirling the pan occasionally. Brush crystals from the sides with a pastry brush dipped in water. Do not stir once the syrup is heating and boiling.

Once dissolved and boiling, attach the thermometer and cook until the syrup reaches 300°F (150°C). Swirl gently to even out heating. Be ready to work immediately.

Remove from heat. Immediately add 1 tbsp baking soda and whisk rapidly and thoroughly, including near the edges and bottom, just until evenly incorporated. The syrup will foam dramatically; do not overmix.

Immediately scrape into the lined pan. Do not spread, press, or tap the foam. Leave at room temperature until completely cool and firm throughout, generally around 60-90 minutes or longer for a thick slab.

Lift out the hardened honeycomb and peel off parchment. Use the tip of a sharp knife to carefully crack into irregular pieces on a dry cutting board. Avoid crushing the bubbles.

Melt and temper 300 g bittersweet chocolate using a reliable method. Keep it free of water and maintain its working temperature. For dark chocolate, this is commonly around 88-90°F (31-32°C), depending on the chocolate.

Partially dip or brush honeycomb pieces in tempered chocolate. Place on parchment and immediately sprinkle with flaky sea salt. Partial dipping keeps the honeycomb structure visible.

Allow the chocolate to set in a cool, dry room. Do not refrigerate or blast-freeze: condensation can make honeycomb sticky.