You can see the more detailed grapefruit curd instructions linked above.  In a small saucepan, whisk together the sugar and egg until smooth, then add the grapefruit juice and zest and turn the stove on medium heat (not too high!).  Whisk continually for at least 5 minutes to thicken.  The mixture should be thick enough to coat the back of a spoon. 

Remove the pan from the heat.  Add one tablespoon of butter at a time, whisking until each has fully melted before adding the next.  Once all of the butter has been whisked in, transfer the curd to an air-tight container and refrigerate until completely cooled (it will keep thickening).

Preheat the oven to 355 F/180C.  Grease and line two 8-inch round cake tins.

Mix all the cake ingredients together using your stand mixer and the paddle attachment. 

Divide the mixture equally between the two tins and bake on the middle shelf of the oven for 25-30 minutes.  Allow to cool in the tins for five minutes, then turn out onto a wire rack.

Meanwhile, for the grapefruit sugar syrup, place the grapefruit juice and sugar in a small saucepan and bring to a boil.  Boil for around five minutes, or until the syrup begins to thicken.  Remove from the heat and cool slightly.

While the cakes are still warm, pierce the surface with a toothpick all over and drizzle the syrup evenly over both cakes (I definitely didn't use all the syrup).  Allow to cool completely.

For the mascarpone cream, whisk the ingredients together until thick and airy, but not too stiff; a stand mixer works well but isn't necessary.

Once it's cooled, place the bottom layer of the cake onto a serving plate.  Spread half the mascarpone cream on the cake, and spoon a generous amount of grapefruit curd over that (some may spill over the edges).

Place the other cake on top and spread the remaining icing on top of the cake.