Beat the softened butter until light and creamy.
Add the light brown sugar, caster sugar, and vanilla extract. Beat until smooth and fluffy.
Add the plain yogurt and mix until well combined.
In a separate bowl, whisk together the all-purpose flour, cornstarch, baking soda, and salt.
Gradually add the dry ingredients to the wet ingredients. Mix until just combined.
Fold in the M&M's candies until evenly distributed throughout the dough.
Scoop equal portions of dough onto a parchment-lined baking sheet, leaving about 2 inches between each cookie. Press a few extra M&M's on top of each dough ball if desired.
Bake in a preheated 350°F (175°C) oven for 10–12 minutes, or until the edges are lightly golden and the centers are set.
Allow the cookies to cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely before serving.
