Topping - 
3/4 cup brown sugar
3/4 cup all purpose flour
2 teaspoons cinnamon
4 tablespoons unsalted butter, melted
Cake - 
2 cups flour
1 teaspoon baking powder
1/4 teaspoon baking soda
1/2 teaspoon salt
6 tablespoons unsalted butter at room temp
1/2 cup sugar
1/4 cup brown sugar
2 large eggs at room temp
1 teaspoon pure vanilla extract  
1/2 cup sour cream
1/2 cup Bailey's Irish cream
Glaze:
1/2 cup powdered sugar
2 tablespoons irish cream

Heat oven to 350 degrees.  Line a 8 x 8 inch baking pan with parchment paper or lightly grease pan with cooking spray

For the topping mix together the brown sugar, flour and cinnamon.  Next, add the melted butter and stir til combined.  Set aside.

For the cake whisk together flour, baking powder and soda, and salt.  Set aside.

Mix butter and sugars together until light and fluffy, about 3 minutes.  With the mixer on medium high add the eggs one at a time, beating each until incorporated.  Add vanilla and sour cream.  Mix til combined.

With mixer on low add half of the flour mixture and mix for a few seconds.  Add the Irish cream and mix for a few seconds.  Add remaining flour and mix until just combined.  Be careful not to overmix.  

To assemble - pour half of the batter into your prepared pan and spread evenly.  Top with half of the topping.  Repeat both layers.

Bake for 40 minutes or until a toothpick inserted in center comes out clean.  Cool on rack for 15 minutes.

For the glaze whisk together powdered sugar and Irish cream.  Drizzle over the cake.  If you want the glaze thicker use less Irish cream.  And, the glaze is optional.

Hope you try it and enjoy the really subtle Irish cream flavor in this coffee cake.  It was win in this house :)