Place the rack in the middle of the oven and preheat it to 180 °C / 350 °F. Line a large baking sheet with parchment paper.
Mix the softened butter, icing sugar, and vanilla in a bowl using an electric mixer. Mix for 2 minutes at medium speed until the mixture is combined and smooth. Add the flour, custard powder, baking powder, and a pinch of salt. Quickly mix to combine, then using a spatula and your hands, knead to get smooth cookie dough.TipFeel free to use a stand mixer fitted with a paddle attachment.
coat in sesame seeds
Add sesame seeds to a small bowl. Using a spoon, scoop out around 15g /0.5 ounce balls. Coat half of the cookie balls in sesame seeds and place them on the prepared baking sheet. Keep the rest of the cookie balls uncoated and put them on the baking sheet too. Using the tines of a fork, gently press the cookies against the baking sheet. Make sure there's enough space in between each cookie.
bake in the oven
Place the baking sheet with the cookies onto the preheated oven. Bake for 10 - 12 minutes at 180 °C / 350 °F.
spread with caramel
When the cookies are baked, remove them from the oven and set them aside to cool. Heat the caramel. Open the glass jar, and place it in a saucepan filled with warm water, so that the water covers about half of the glass. Leave the jar in the water for a minute for the caramel to get smooth. Spread the caramel over the sesame seed coated cookies and close with the uncoated cookie. Serve or store.TipFeel free to use Dulce de Leche too. In that case, there's no need to heat it.