Whisk together the flour, baking powder and salt in a bowl and set aside.
Beat the butter and sugar with an electric mixer until combined, then add the eggs and mix until combined. Reduce the speed to low and add the flour, mixing until fully combined. Then add the milk and vanilla and continue beating until just incorporated.
Spray a regular-sized doughnut pan with cooking spray. Place the batter in a piping bag fitted with a round tip and fill the pan with the batter. Bake at 350F for 10 minutes. Cool in the pan for 10 minutes, then transfer to a wire rack.
Combine 1 tbsp sugar and 1 tbsp water in a small dish and brush onto half of the doughnuts. Sprinkle sesame seeds on top.