In a heatproof bowl, slowly melt ⅔ of the white chocolate in the microwave for 20secs at a time, stirring between bouts until smooth.
Using a teaspoon, spoon a small amount the melted chocolate into each of the popsicle moulds. Gently spread out the chocolate, with a small spoon or food-safe brush, ensuring you cover the sides up to the edges. Tap your mould on the surface a few times to remove any air bubbles.
Push the popsicle sticks partway into the moulds (about ½ inch / 1-2 cm), then put in the fridge for 10mins to set.
Optional step: brush any of the remaining ⅔ of melted white chocolate in the edges and up the sides of your popsicle mould to reinforce the chocolate shell. Let this set in the fridge for another 5mins.
Take the moulds from the fridge and fill them with cakesicle mix to just below the edges, pressing in gently to smooth the top.
Push the popsicle sticks in until they sit roughly ⅔ of the way into the mould, pressing the cakesicle mix down if needed. Put the moulds back in the fridge.
In a heatproof bowl, melt the remaining ⅓ of white chocolate in the microwave, 20secs at a time, stirring until smooth.
Remove the moulds from the fridge and spoon the melted chocolate over them, covering the cakesicle mix completely.
Place your moulds back into the fridge for 10mins or until set, then carefully pop each cakesicle out of the mould. Run a sharp knife around the edges to clean up the sides if needed.
Add any optional decorations to the top of your cakesicles, such as a drizzle of warmed biscoff spread (microwave for 10secs) and crushed biscoff cookies.