Preheat the oven to 180°C/160°C fan/350°F/gas mark 4. Line two baking sheets with baking parchment.
Whisk together the butter, the light brown soft sugar and the caster/granulated sugar with an electric mixer until well combined.
Whisk in the peanut butter, vanilla extract and milk until it is smooth and creamy.
Whisk in the baking powder, bicarbonate of soda and salt then add the flour and stir with a spatula until it forms a smooth, stiff dough with no dry patches. If it is too crumbly then you can add a drop more milk as needed.
Roll the dough into golf ball sized balls (I weighed mine and did 30g per ball) and place them spaced apart on the lined baking sheets.
Press the tines of a fork into the top of each ball of dough to flatten them, first in one direction then in the other to form a criss-cross pattern.
Bake the cookies for 10-12 minutes until pale golden if you like them on the softer side, or for 15-20 minutes until they are a deeper golden if you like them more crunchy (I go for 15 minutes). They will be quite soft when they come out of the oven but will firm up as they cool.
Let them cool on the trays for 5 minutes then transfer them to a wire rack to cool completely. Once cold store in an airtight container at room temperature.