Pre-heat the oven to 170°C and line 2 x muffin tins (1 x 12 hole and 1 x 6 hole) with cupcake cases.
Beat together the sugar and butter until light and fluffy.
Add the eggs one at a time, beating on a low speed until fully combined.
In a measuring jug whisk together the whole milk, sour cream and vanilla extract and set aside for a moment.
Whisk together the flours, baking powder, bicarbonate of soda and salt in a large mixing bowl until completely combined.
Add the flour mix alternately with the milk mixture, adding the flour in three additions and the milk in two (begin and end with the flour), scrape down the sides of the bowls as needed and mix until just combined.
Finally add in the mincemeat, mixing in until evenly distributed throughout the batter.
Divide the batter between the cupcake cases and bake for 18 minutes. Remove the cupcakes immediately onto a wire rack to cool before piping on the buttercream.
Heat the egg whites and caster sugar in a bain marie (or a bowl set over simmering water), stirring constantly until the sugar has dissolved and the temperature has reached 71°C.
Remove the egg whites and sugar from the heat and pour into a stand mixer with whisk attachment. Whisk until the mixture forms stiff peaks.
Change the attachment to a paddle attachment. On a low speed add the butter slowly cube by cube. When you have almost added all the butter the mixture will look curdled. Do not fret – this is supposed to happen and just means you are nearly done. Just continue to add all the butter. Once the butter is totally incorporated the frosting will miraculously become a smooth velvety consistency or if not just leave the mixer beating until it does. It will.
Add the salt, vanilla extract and brandy and mix until thoroughly combined.
Pipe the buttercream on top of the cupcakes. I used a Wilton 1M piping tip and piped the buttercream in a circle from the centre swirled to the outer edge of the cupcake. I then cut out the holly leaf using a teeny tiny leaf cutter from green sugarpaste and I piped on the red berries with a no.2 piping tip with a bit of the brandy buttercream tinted red. To make life easier you can buy these decorations ready made from good cake decoration suppliers.