Mix the agave syrup, liquid sweetener, yolks, cornstarch, and salt in a heatproof bowl.
In a saucepan, mix the milk, yoghurt, and vanilla extract on medium heat until hot.
While whisking, slowly stream about half of the hot milk/yoghurt liquid in the yolk mixture. Pour this tempered yolk mixture into the saucepan with the rest of the milk/yoghurt mixture. Cook over medium-low heat for 4 to 8 minutes, stirring frequently, until bubbling and thickened like pudding.
Turn off the heat, mix in the butter. Strain through a fine-mesh sieve into a bowl, discarding any clumps caught in the sieve. Cover the pastry cream with plastic wrap to prevent a skin from forming. Refrigerate until cold.
Phyllo Pastry
Defrost the phyllo pastry. Put a damp tea towel on it so it doesn't dry out.
Melt the butter. Take one sheet of phyllo pastry, carefully spread it with butter, place another sheet on top and repeat the process till you have layered 5 sheets.
Create 4 of these layered phyllo pastries.
Toppings
Preheat the oven at 220°C.
Put the custard in the center of the sheet. Layer strawberries on top of it. Close the phyllo pastry.
Place them on a baking tray and bake for 20 minutes or till golden.