Preheat the oven to 180ºC.
In a medium bowl, use an electric hand whisk to cream the butter and sugars until light and fluffy.
Add the egg and peanut butter, whisking until completely incorporated.
Pour in the flour and bicarbonate of soda. Use a wooden spoon or spatula to mix the ingredients until a dough forms.
Using a cookie scoop or a tablespoon, measure out 3 tablespoons of dough for each cookie, forming them into golf ball-sized shapes. Leave 2 inches of space between each cookie on the baking tray.
Gently flatten each cookie with your fingers, then use a fork to create a crisscross pattern on the dough.
Bake in the preheated oven for about 10 minutes, until the cookies are light golden and just set in the middle. Let them cool on the baking tray for about 5 minutes before transferring to a cooling rack.
Melt the chocolate in the microwave or in a bowl over a pan of simmering water. Once melted, drizzle the chocolate over the cookies using a spoon and sprinkle with the chopped peanuts.