Mix the flour with baking powder. In a mixer, cream the butter and sugar until light. Add the eggs, creaming the mixture constantly, and finally mix in the lemon zest and juice. In the mixer bowl, add flour and pour in kefir Mix everything at low speed until smooth. Line the muffin tin with cupcake liners. Pour the dough into each one up to 3/4 of the height. Bake in an oven preheated to 374°F (190°C) for 25 minutes. Set aside to cool.
Add the Lauretta 50% exotic fruit filling to the whipped mascarpone and mix everything. Transfer the mixture to a pastry bag and decorate the tops of the muffins.