Preheat the oven to 180˚C/350˚F and grease the tart tin.
In a food processor, blitz the biscuits until they resemble coarse bread crumbs.
If you have a feeding tube, add the butter as you continue running the food processor, otherwise, just add the butter and blitz again to mix thoroughly.
Add the demerara sugar, if using, mix throroughly.
Take this crumb mix and press into the tart tin, following the slanted edges and packing everything in.
Bake in the oven for 10 minutes then take it out and let cool slightly while you make the filling. When you take it out, while it's still hot and pliable, use a teaspoon or rounded knife to push/ease the pastry back to shape if it needs it. I like to push the base edges back to give the filling more room.
Using a hand whisk, beat the egg yolks for about a minute, add both types of milk and whisk until blended. Go easy as you don't want too many bubbles.
Add the lime juice gradually as you continue to whisk gently. You'll notice an immediate change in texture as the custard thickens and gives off a sour smell. Don't worry, it's supposed to.
Gently pour into the prepared crust and bake for 15 to minutes. When done, the filling should have a slight wobble, if you're not sure, leave it in for only a minute or two more, no longer, as it will start to rise and crack, you don't want to go that far.
Let cool, then refrigerate for at least 6 hours or overnight as I like to do.
Take it out and rest at room temperature for about 15 minutes before serving for the best texture, this will give the butter-filled base a chance to relax. Serve with some whipped cream or a scoop of ice cream.