3/4 cup butter, softened
3/4 cup sugar
1 teaspoon vanilla
1 1/2 cups all-purpose flour
3/4 cup unsweetened cocoa
1/4 teaspoon salt
1 1/2 cups powdered sugar
1/4 cup butter, softened
2 to 3 teaspoons Irish cream liqueur**
Powdered sugar, if desired
Combine 3/4 cup butter and sugar in large bowl. Beat at medium speed, scraping bowl often, until creamy. Add egg and vanilla; continue beating until well mixed. Reduce speed to low; add flour, cocoa and salt.
Beat until well mixed. Divide dough in half; shape each half on lightly floured surface into 8-inch long square log with 1 1/2-inch sides.
Wrap in plastic food wrap; refrigerate until firm (2 hours or overnight).
Heat oven to 325°F. Cut logs into 1/4-inch square slices with sharp, serrated knife. Place squares 1 inch apart onto ungreased cookie sheets.
Bake for 6 to 8 minutes or until set.
Combine powdered sugar and 1/4 cup butter in small bowl. Beat at low speed, adding enough liqueur for desired spreading consistency.
Spread about 1 teaspoon filling on cookie, top with second cookie. Press together gently. Sprinkle with powdered sugar, if desired.
**Substitute 2 to 3 teaspoons very strong brewed coffee.
TIP: Make sure not to slice the cookie dough too thick or the cookies will not be crisp.
Make Ahead: Prepare cookies as directed; cool completely. Freeze in airtight container up to 1 month. When ready to use, thaw cookies completely. Prepare filling and assemble cookie sandwiches as directed.