Put flour, sugar, butter, vanilla and cinnamon into the food processor and process for 1 minute.
Add yolk and pulse until just combined.
Tip mixture onto greaseproof paper or glad wrap and roll into a sausage round 4cm in diameter. Wrap in glad wrap and chill in fridge until solid.
Preheat oven to 180°C. Line baking sheet with baking paper.
Cut dough into 3-4mm discs (thinner than in my above photo, to achieve a better biscuit to dulce de leche ratio!)
Pour sesame seeds onto a plate and press one side of each biscuit into the sesame seeds.
Place biscuits on baking sheet and bake for 10 mins or until lightly golden.
Once completely cool, spread half the biscuits with dulce de leche and gently sandwich the biscuits together.