Preheat the oven to 350°F or about 170°C (160°C for fan ovens).
Grease the bottom and sides of a 7 or 8-inch pan with butter. Line the bottom with baking parchment and then grease the parchment as well. A pan with a loose bottom makes the squares easier to cut and serve.
Put the mincemeat in a small bowl and stir in the rum, brandy, Cointreau or Grand Mariner. Set aside.
Combine the flour with the butter in a medium bowl and cut the butter into the flour until mixture resembles fine crumbs. You can use a food processor for this or do it by hand with an old-fashioned pastry blender.
Add the oats, baking soda, salt and sugar to the butter and flour mixture and mix thoroughly.
Press half of this mixture into the prepared pan.
Put the pan in the freezer and put the remaining oat mixture in the fridge. Chilling the pan in the freezer makes it easier to spread the boozy mincemeat over the bottom crust.
After 15 minutes, remove the square pan from the freezer and the remaining oat mixture from the fridge.
Carefully spread the boozy mincemeat over the top of the oat mixture in the square pan.
Gently sprinkle the remaining oat mixture evenly over top of the mincemeat. It does not cover the mincemeat completely, there are a few gaps.
Bake in the oven for about 25 to 30 minutes or until the squares are lightly golden brown. Be sure to keep an eye on the pan in the last minutes of baking so the Mincemeat Oat Squares do not burn.
Remove from the oven and allow to cool completely at room temperature., then place the traybake in the fridge for at least an hour before cutting. I often leave them overnight.
Leftovers will keep in a sealed container in the fridge for several days.