Preheat an oven to 200°c/180°c fan (392°f/356°f).
Line a baking tray with parchment paper then set aside.
Sift the flour, baking powder, bicarbonate of soda & salt into a large mixing bowl then add in the diced butter.
Using your hands, rub the butter into the flour until the mix resembles breadcrumbs but with some large pieces still visible.
Next, crack the egg into a separate bowl then add in the sour cream. Beat together with a fork then add into the flour.
Mix into a rough dough with a spatula or your hands then tip out onto a lightly floured work surface.
Shape the dough into a ball then press down with your hands to make a 20cm wide circle.
Using a sharp knife, cut the dough into 8 triangles, place them onto the lined baking tray then chill in the fridge for 15 minutes.
Brush the top of each scone with milk then sprinkle over the sesame seeds.
Bake in the preheated oven for 16-18 minutes, until a deep, golden brown then transfer to a cooling rack & leave to cool completely.