Preheat the oven and prepare the tin

• Preheat the oven to 180 °C.
• Line a square baking dish with baking paper.

Cream the butter and sugars

• Beat the softened butter with the light brown sugar and caster sugar until pale and fluffy.

Add the egg and vanilla

• Beat in the egg and vanilla extract until fully combined.

Mix in the dry ingredients

• Fold in the plain flour, baking soda and salt until a soft dough forms.

Add the M&M's

• Stir the M&M's through the dough, then press the dough evenly into the lined dish.
• Scatter a few extra M&M's over the top.

Bake

• Bake until golden at the edges and just set in the middle.
• Leave to cool completely in the dish.

Slice and serve

• Lift out of the dish and cut into slices.