Preheat oven to 175C / 350F (with or without fan assistance is fine). Toast the black and white sesame seeds for 5 to 10 minutes until fragrant. Alternative, toast them in a small frying pan until fragrant. Remove from heat and set aside to cool completely.
Turn over a baking sheet (minimum 9-inch big) and place a baking parchment or silpat on top. Lightly oil the baking parchment or silpat. Set aside.
Place sugar in a small pot. Add in water. Do not stir. Place the pot over medium heat and heat until the sugar has dissolved, swirling the pan occasionally. Allow the mixture to boil and bubble for 5 minutes, until it turns a light amber colour - do not stir the mixture!
Once the caramel is light amber in colour, remove from heat and working quickly, stir in the butter with a silicone spatula until mixture is smooth and any frothing has subsided. Stir in the sesame seeds and immediately pour the mixture onto the baking parchment / silpat, spreading it out as thinly as possible.
Let the brittle cool and harden completely, then break into small pieces. Store them in an airtight container at room temperature.