Preheat oven to 375°F (190°C). Butter the sides and bottom of a 9"x 13"x 2" baking pan.
Toss the chocolate chips with the liquor in a small bowl. Sprinkle 2 tablespoons of flour over the chips and toss until coated. Set aside.
Heat 1 & 1/4 cups (300ml) water to boiling. Place the oats and butter in a large bowl. Pour the boiling water over the oat mixture. Wait 30 seconds, then stir to moisten the oats and melt the butter. Set aside for 25-30 minutes.
Whisk the eggs, sugars, salt, baking soda, baking powder, and cinnamon. Fold in the oat mixture, stirring until well combined. Fold in the remaining flour, and then stir in the chocolate chips. Pour the batter into prepared pan.
Bake 40-45 minutes until a toothpick inserted into the center of the cake comes out clean. Cool on a wire rack for 30 minutes.
To make the frosting:
Beat the butter until smooth. Add the cream cheese and beat until combined. Beat in the confectioners' sugar and vanilla extract until smooth (about 1 minute). Cover the bowl and refrigerate for at least 30 minutes.
Spread a thin layer of frosting over the cake. Chill for 15 minutes before serving.
Store the covered cake in the refrigerator for up to 3 days.
Notes
*A liqueur (like Kahlua or Frangelico) works well too.
Recipe slightly adapted from Baked Explorations.
Store tightly covered in the refrigerator up to 3 days.