Heat oven to 180C/160C fan/gas 4. Line a 20 x 30cm baking or roasting tin with baking parchment. Beat together the butter, flour, sugar, baking powder, eggs, yogurt and vanilla. Stir in the chopped cherries and snipped marshmallows. Scrape into the tin and bake for 25-35 mins until a skewer poked in comes out clean. Cool.
To decorate, put the marshmallows into a pan with the milk and gently melt, stirring. Scrape into a bowl and cool, stirring occasionally, until thick enough to spread. Spread over the cake, dot over the halved cherries and slice into 15 squares.