Preheat the oven to 350F/175C and lightly grease two six-hole donut pans or one six-hole donut pan and one mini 12-hole donut pan.
In a medium bowl, stir together the flour, cocoa powder, baking powder and sugar.
Pour the vegetable oil, vanilla extract, yogurt, milk, lemon juice, food colouring and egg into the bowl with the dry ingredients and beat until everything comes together.
Pour the donut batter (it will be thin) into a large piping bag and pipe into the donut pans, filling each mould ¾ of the way full.
Bake in the preheated oven for around 10 minutes, until the tops are springy to the touch.
Remove the donuts from the oven and leave to cool in the pan on a wire rack for about 2 hours, or until at room temperature.
To make the icing, use a fork to beat the cream cheese and condensed milk together.
Spread the icing evenly on top of the cooled donuts and sprinkle with cute candy hearts.