Preheat oven to 190C/375F (170C fan) with one shelf in the middle and another under it.

Use a bowl at room temp, not ice cold glass/metal (butter will go hard).

Place butter in bowl and beat until smooth - about 30 seconds on speed 5.

Add sugar and beat for 45 sec to 1 min until light and fluffy.

Add yolks, beat until fully incorporated (~20 sec).

Add vanilla and glucose, beat until incorporated (~15 sec).

Add flour, baking powder and cocoa. Stir with wooden spoon until incorporated (will be a stiff dough).

Mix through chocolate chips.

Scoop / roll 12 balls a touch smaller than a golf ball, then flatten to 1cm / 2/5" thick on baking trays (2 trays).

Bake both trays for 10 minutes.

Take top tray out, move tray underneath to top shelf and bake a further 1 minute then remove. The cookies will look ever so slightly undercooked in the centre - they will finish cooking on the tray (this is how we make them chewy!)

Leave cookies to cool on the tray for 20 minutes, then transfer to cooling rack.

Allow to cool fully before serving (becomes chewy when cools, even better the next day!)