Preheat oven to 350 degrees.
Mix flour with baking powder and salt.
Melt butter with chocolate over very low heat or over hot water.
Beat eggs with sugar until light.
Stir in chocolate mixture and 1/4 cup Kahlua.
Add flour mixture and mix well. Stir in walnuts.
Turn into greased 9 inch square pan lined with greased parchment paper.
Bake in center of oven for 30 minutes.
Top will spring back when touched lightly in center, and edges will begin to pull away from pan.
Do not overbake.
Remove from oven and cool in pan.
When cool, brush top with 1 tablespoon Kahlua.
Let stand until thoroughly cold before cutting.