1) Preheat the oven to 180°C.
2) Butter and flour a 12-cup muffin pan, or use paper cases. In a large bowl, combine the flour, baking powder and three-quarters of a teaspoon of salt.
3) In the bowl of an electric mixer, beat the sugar and butter until light and fluffy. Beat in the eggs, one at a time then beat in the vanilla extract. With mixer on low, beat in the flour mixture in two parts, alternating with milk.
4) Divide the batter among the prepared muffin cups. Bake until a toothpick inserted in center of a cupcake comes out clean, about 20 minutes. Let cool in the pan for five minutes. Remove the cupcakes from the pan and transfer to a wire rack to cool completely.
5) Using a paring knife, cut a 2cm deep piece from the top of each cupcake. Save the cut pieces. Fill each hole with one teaspoon of cajeta and replace the cut out pieces. Frost the cupcakes with Mexican vanilla buttercream frosting.
6) For the frosting: using an electric mixer on low speed, beat the butter and sugar until smooth. Add the salt, vanilla extract and crema. Increase the speed to high and beat until fluffy, about three minutes, adding extra Mexican crema, as needed, to make the frosting spreadable.