Place butter bar in a jug and microwave for about 50 seconds, until completely melted. Alternatively melt it in the pot over low fire. With a pastry brush grease baking tray, using a bit of melted butter, leave aside. In a mixer bowl combine all dry ingredients: flour, vanilla seeds, sugar and baking powder. Add also milk and eggs and turn the mixer on. Gradually pour in melted butter and mix a while longer. Stir in walnuts and transfer batter to the baking tin. Bake in the the oven preheated to 180°C (356°F) for about an hour. Let the cake to cool down before cutting it into two layers. In meantime prepare the buttercream by whisking all of the ingredients together. Once the cake is cooled down, spread buttercream on the bottom layer and put back the top layer. Place in a fridge for at least 30 minutes before sharing.
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