Preheat oven to 180°C (Gas mark 4/350°F). Line a 12 hole muffin tin with 12 muffin cases.
Place the butter into a mixing bowl and whisk together with the caster sugar until light and fluffy. Break the eggs into a bowl and lightly whisk with a fork. Add the eggs a little at a time to the butter and sugar mix, whisking in between.
Sift the flour on top, and using an electric whisk on a low speed, gently blend the ingredients together. Add the milk and vanilla extract and blend again. Increase the mixer speed and whisk for a few more seconds until well mixed.
Divide the cake mixture between the 12 cupcake cases and smooth over the tops with the back of a spoon. Bake for 20-25 minutes until risen, golden and springy to the touch.
Once the cakes are cooked, transfer to a wire rack to cool.
Whilst the cakes are cooling, make the buttercream icing:
Whisk the butter with an electric whisk until light and fluffy. Sift in the icing sugar, a little at a time, until completely combined. Add the vanilla extract and whisk again.
Add the milk in small splashes until the icing is at the correct consistency for piping.
Colour the fondant icing - leave 200g plain white, colour 250g blue and colour 150g red.
Roll out the blue fondant icing on a worktop dusted with icing sugar. Using a round circle cutter the same size as the top of your cupcakes, cut out 12 circles for the base of the toppers.
Once the cakes are cool, pipe a small flat swirl or spread buttercream over each one. Carefully place a blue fondant circle on top, pressing gently around the edges to mould it to the top of the cake.
Roll out the white fondant. Cut circles from the white fondant using the same circle cutter as before. Cut a thick cross from each white circle and add to the top of each cake, using water to fix them in place if needed.
Cut diagonal cross pieces from the remaining white fondant circle pieces and add them to the cakes.
Roll out the red fondant. Cut circles from the red fondant using the same circle cutter as before. Cut a thinner cross from each red circle and add to the top of each cake.
Cut thin diagonal cross pieces from the remaining red fondant circle pieces and add them to the cakes to finish.