Heat the oven to 350°F (175°C). Grease a 9-inch square baking pan and line it with parchment, leaving enough paper over the sides to lift out the cooled cake.

Make the streusel by stirring together the flour, brown sugar, cinnamon, and salt. Add the melted butter and stir until the mixture forms coarse, buttery crumbs.

Whisk the cake flour, baking powder, baking soda, and salt in a separate bowl. Keeping the dry ingredients evenly combined helps the cake rise consistently.

Beat the softened butter and granulated sugar for 2–3 minutes, until light and fluffy. Add the eggs one at a time, beating well after each addition, then mix in the vanilla and optional Irish whiskey.

Stir the sour cream and cooled brewed coffee together in a small bowl. Add the dry ingredients to the butter mixture in two or three additions, alternating with the sour-cream mixture, and mix only until combined.

Spread half of the batter in the prepared pan. Sprinkle roughly one-third of the streusel over the batter, add the remaining batter, and finish with the rest of the streusel.

Bake for 35–45 minutes, until the center springs back lightly and a toothpick inserted into the middle comes out with a few moist crumbs. Avoid waiting for a completely dry toothpick, since that can indicate an overbaked cake.

Cool the cake in the pan for 15–20 minutes, then use the parchment to lift it onto a rack and let it cool further. Whisk the confectioners’ sugar with cooled coffee and the optional whiskey or Irish cream, then drizzle the glaze over the cake.