Preheat the oven to 180C/350F and grease and line an 8x8 inch square pan.
Make the streusel first: Place oats, sugar, flour and butter into a medium sized bowl and rub together until it starts to combine. Then add in the vanilla extract and continue rubbing between your fingers until you have a coarse breadcrumb texture. Some larger and smaller lumps are fine! Leave to one side.
Make the cookie layer: Place mint chocolate chips/chunks, flours, bicarb and salt into a medium sized bowl and stir until all the chocolate chunks are coated in flour. Leave to one side.
Place butter and sugars into a large bowl, or the bowl of a stand mixer and beat until light and fluffy, 2-3 minutes on medium-high speed.
Add in the egg, vanilla extract and peppermint extract, scraping down sides as necessary and mix until well incorporated and smooth, 1 minute on med-high speed.
Tip in the flour and chocolate mixture and mix on low until a soft dough forms.
Tip the cookie dough into your prepared pan and press into a level layer with either your hands or the back of a wooden spoon/spatula. (I used my hands, much easier!)
Sprinkle the streusel topping all over the cookie dough (there's plenty to cover it!) and press it into the cookie dough ever so slightly.
Place in the oven for 25-30 minutes until risen around the edges and slightly golden.
It can be a little hard to tell if it's done; it was golden and firm around the edges, the topping was ever so slightly coloured and the centre was still pretty soft and under done. (I made this twice and the first time I over did it as I thought it wasn't cooked in the centre, but underdone will be fine once cooled as it keeps cooking!)
Leave to cool in the pan completely, before transferring to a cutting board to slice into 9-12 bars.
Mint Chocolate Streusel Cookie Bars will keep in an airtight container, at room temperature for 5 days.